.NET: Add ITextSearch<TRecord> with LINQ filtering and deprecate legacy ITextSearch (#10456) (#13179)

# .NET: Add LINQ-based ITextSearch<TRecord> interface and deprecate
legacy ITextSearch (#10456)

## Summary

This PR implements **Option 3** from the architectural decision process
for Issue #10456: introduces a new generic `ITextSearch<TRecord>`
interface with type-safe LINQ filtering while maintaining the legacy
`ITextSearch` interface marked as `[Obsolete]` for backward
compatibility.

**Zero breaking changes** - existing code continues working unchanged.

## What Changed

### New Generic Interface (Recommended Path)
```csharp
public interface ITextSearch<TRecord>
{
    Task<KernelSearchResults<string>> SearchAsync(
        string query, 
        TextSearchOptions<TRecord>? searchOptions = null, 
        CancellationToken cancellationToken = default);
    
    // + GetTextSearchResults and GetSearchResults methods
}

// Type-safe LINQ filtering with IntelliSense
var options = new TextSearchOptions<CorporateDocument>
{
    Filter = doc => doc.Department == "HR" && 
                   doc.IsActive && 
                   doc.CreatedDate > DateTime.Now.AddYears(-2)
};
```

**Benefits:**
- ✅ Compile-time type safety
- ✅ IntelliSense support for property names
- ✅ Full LINQ expression support
- ✅ No RequiresDynamicCode attributes
- ✅ AOT-compatible (simple equality/comparison patterns)

### Legacy Interface (Deprecated)
```csharp
[Obsolete("Use ITextSearch<TRecord> with LINQ-based filtering instead. This interface will be removed in a future version.")]
public interface ITextSearch
{
    Task<KernelSearchResults<string>> SearchAsync(
        string query, 
        TextSearchOptions? searchOptions = null, 
        CancellationToken cancellationToken = default);
}

// Legacy clause-based filtering (still works)
var options = new TextSearchOptions 
{ 
    Filter = new TextSearchFilter().Equality("Department", "HR") 
};
```

**Migration Message:** Users see deprecation warning directing them to
modern `ITextSearch<TRecord>` with LINQ filtering.

## Implementation Details

### Dual-Path Architecture

`VectorStoreTextSearch<TRecord>` implements both interfaces with
independent code paths:

**Legacy Path (Non-Generic):**
```csharp
async IAsyncEnumerable<VectorSearchResult<TRecord>> ExecuteVectorSearchAsync(
    string query, TextSearchOptions options)
{
    var vectorOptions = new VectorSearchOptions<TRecord>
    {
        #pragma warning disable CS0618 // VectorSearchFilter is obsolete
        OldFilter = options.Filter?.FilterClauses != null 
            ? new VectorSearchFilter(options.Filter.FilterClauses) 
            : null
        #pragma warning restore CS0618
    };
    // ... execute search
}
```

**Modern Path (Generic):**
```csharp
async IAsyncEnumerable<VectorSearchResult<TRecord>> ExecuteVectorSearchAsync(
    string query, TextSearchOptions<TRecord> options)
{
    var vectorOptions = new VectorSearchOptions<TRecord>
    {
        Filter = options.Filter  // Direct LINQ passthrough
    };
    // ... execute search
}
```

**Key Characteristics:**
- Two independent methods (no translation layer, no conversion overhead)
- Legacy path uses obsolete `VectorSearchFilter` with pragma
suppressions (temporary during transition)
- Modern path uses LINQ expressions directly (no obsolete APIs)
- Both paths are AOT-compatible (no dynamic code generation)

## Files Changed

### Interfaces & Options
- `ITextSearch.cs`: Added `ITextSearch<TRecord>` interface, marked
legacy `ITextSearch` as `[Obsolete]`
- `TextSearchOptions.cs`: Added generic `TextSearchOptions<TRecord>`
class

### Implementation
- `VectorStoreTextSearch.cs`: Implemented dual interface pattern (~30
lines for both paths)

### Backward Compatibility (Pragma Suppressions)
Added `#pragma warning disable CS0618` to **27 files** that use the
obsolete interface:

**Production (11 files):**
- Web search connectors (Bing, Google, Brave, Tavily)
- Extension methods (WebServiceCollectionExtensions,
TextSearchExtensions)
- Core implementations (TextSearchProvider, TextSearchStore,
VectorStoreTextSearch)

**Tests/Samples (16 files):**
- Integration tests (Agents, AzureAISearch, InMemory, Qdrant, Web
plugins)
- Unit tests (Bing, Brave, Google, Tavily)
- Sample tutorials (Step1_Web_Search, Step2_Search_For_RAG)
- Mock implementations

### Tests
- Added 7 new tests for LINQ filtering scenarios
- Maintained 10 existing legacy tests (unchanged)
- Added `DataModelWithTags` to test base for collection filtering

## Validation Results

- ✅ **Build**: 0 errors, 0 warnings with `--warnaserror`
- ✅ **Tests**: 1,581/1,581 passed (100%)
- ✅ **Format**: Clean
- ✅ **AOT Compatibility**: All checks passed
- ✅ **CI/CD**: Run #29857 succeeded

## Breaking Changes

**None.** This is a non-breaking addition:
- Legacy `ITextSearch` interface continues working (marked `[Obsolete]`)
- Existing implementations (Bing, Google, Azure AI Search) unchanged
- Migration to `ITextSearch<TRecord>` is opt-in via deprecation warning

## Multi-PR Context

This is **PR 2 of 6** in the structured implementation for Issue #10456:

- **PR1** ✅: Generic interfaces foundation
- **PR2** ← YOU ARE HERE: Dual interface pattern + deprecation
- **PR3-PR6**: Connector migrations (Bing, Google, Brave, Azure AI
Search)

## Architectural Decision

**Option 3 Approved** by Mark Wallace and Westey-m:

> "We typically follow the pattern of obsoleting the old API when we
introduce the new pattern. This avoids breaking changes which are very
disruptive for projects that have a transient dependency." - Mark
Wallace

> "I prefer a clean separation between the old and new abstractions.
Being able to obsolete the old ones and point users at the new ones is
definitely valuable." - Westey-m

### Options Considered:

1. **Native LINQ Only**: Replace `TextSearchFilter` entirely (breaking
change)
2. **Translation Layer**: Convert `TextSearchFilter` to LINQ internally
(RequiresDynamicCode cascade, AOT issues)
3. **Dual Interface** ✅: Add `ITextSearch<TRecord>` + deprecate legacy
(no breaking changes, clean separation)

See ADR comments in conversation for detailed architectural analysis.

## Migration Guide

**Before (Legacy - Now Obsolete):**
```csharp
ITextSearch search = ...;
var options = new TextSearchOptions 
{ 
    Filter = new TextSearchFilter()
        .Equality("Department", "HR")
        .Equality("IsActive", "true")
};
var results = await search.SearchAsync("query", options);
```

**After (Modern - Recommended):**
```csharp
ITextSearch<CorporateDocument> search = ...;
var options = new TextSearchOptions<CorporateDocument> 
{ 
    Filter = doc => doc.Department == "HR" && doc.IsActive
};
var results = await search.SearchAsync("query", options);
```

## Next Steps

PR3-PR6 will migrate connector implementations (Bing, Google, Brave,
Azure AI Search) to use `ITextSearch<TRecord>` with LINQ filtering,
demonstrating the modern pattern while maintaining backward
compatibility.

---------

Co-authored-by: Alexander Zarei <alzarei@users.noreply.github.com>

Author: alzarei

dotnet/samples/GettingStartedWithTextSearch/Step1_Web_Search.cs

@@ -1,5 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+#pragma warning disable CS0618 // ITextSearch is obsolete - Sample demonstrates legacy interface usage
+
 using Microsoft.SemanticKernel.Data;
 using Microsoft.SemanticKernel.Plugins.Web.Bing;
 using Microsoft.SemanticKernel.Plugins.Web.Google;

dotnet/samples/GettingStartedWithTextSearch/Step2_Search_For_RAG.cs

@@ -1,4 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
+
+#pragma warning disable CS0618 // ITextSearch is obsolete - Sample demonstrates legacy interface usage
+
 using System.Text.RegularExpressions;
 using HtmlAgilityPack;
 using Microsoft.SemanticKernel;

dotnet/src/IntegrationTests/Agents/CommonInterfaceConformance/AgentWithTextSearchProviderConformance/AgentWithTextSearchProvider.cs

@@ -41,7 +41,9 @@ public abstract class AgentWithTextSearchProvider<TFixture>(Func<TFixture> creat
     public async Task TextSearchBehaviorStateIsUsedByAgentInternalAsync(string question, string expectedResult, params string[] ragResults)
     {
         // Arrange
+#pragma warning disable CS0618 // ITextSearch is obsolete - Testing legacy interface
         var mockTextSearch = new Mock<ITextSearch>();
+#pragma warning restore CS0618
         mockTextSearch.Setup(x => x.GetTextSearchResultsAsync(
             It.IsAny<string>(),
             It.IsAny<TextSearchOptions>(),

dotnet/src/IntegrationTests/Connectors/Memory/AzureAISearch/AzureAISearchTextSearchTests.cs

@@ -1,5 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+#pragma warning disable CS0618 // ITextSearch is obsolete
+
 using System;
 using System.Threading.Tasks;
 using Azure.AI.OpenAI;

dotnet/src/IntegrationTests/Connectors/Memory/InMemory/InMemoryVectorStoreTextSearchTests.cs

@@ -1,5 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+#pragma warning disable CS0618 // ITextSearch is obsolete
+
 using System;
 using System.Threading.Tasks;
 using Microsoft.Extensions.AI;

dotnet/src/IntegrationTests/Connectors/Memory/Qdrant/QdrantTextSearchTests.cs

@@ -1,5 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+#pragma warning disable CS0618 // ITextSearch is obsolete
+
 using System;
 using System.Threading.Tasks;
 using Microsoft.SemanticKernel.Connectors.Qdrant;

dotnet/src/IntegrationTests/Data/BaseTextSearchTests.cs

@@ -1,5 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+#pragma warning disable CS0618 // Type or member is obsolete - Testing legacy non-generic ITextSearch interface
+
 using System;
 using System.Collections.Generic;
 using System.Linq;

dotnet/src/IntegrationTests/Plugins/Web/Bing/BingTextSearchTests.cs

@@ -1,5 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+#pragma warning disable CS0618 // ITextSearch is obsolete
+
 using System.Threading.Tasks;
 using Microsoft.Extensions.Configuration;
 using Microsoft.SemanticKernel.Data;

dotnet/src/IntegrationTests/Plugins/Web/Google/GoogleTextSearchTests.cs

@@ -1,5 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+#pragma warning disable CS0618 // ITextSearch is obsolete
+
 using System.Threading.Tasks;
 using Microsoft.Extensions.Configuration;
 using Microsoft.SemanticKernel.Data;

dotnet/src/IntegrationTests/Plugins/Web/Tavily/TavilyTextSearchTests.cs

@@ -1,5 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+#pragma warning disable CS0618 // ITextSearch is obsolete
+
 using System.Threading.Tasks;
 using Microsoft.Extensions.Configuration;
 using Microsoft.SemanticKernel.Data;