Add support for Retrieve a file upload API

[KoditkarVedant]

Description

Add support for Retrieve a file upload API

Fixes #480

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[Copilot]

Pull Request Overview

This PR adds support for the Retrieve a File Upload API, enabling retrieval of specific file upload details by ID.

Key changes:

• Implemented RetrieveAsync method in FileUploadsClient with validation for file upload ID
• Added request/response models and interface definitions for the retrieve operation
• Included comprehensive unit and integration tests

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Test/Notion.UnitTests/FileUploadsClientTests.cs	Added unit tests for RetrieveAsync method validation and REST client interaction
Test/Notion.IntegrationTests/FileUploadsClientTests.cs	Added integration test that creates and retrieves a file upload
Src/Notion.Client/Api/FileUploads/Retrieve/Response/RetrieveFileUploadResponse.cs	Defined response model extending FileObjectResponse
Src/Notion.Client/Api/FileUploads/Retrieve/Request/RetrieveFileUploadRequest.cs	Defined request model implementing path parameters interface
Src/Notion.Client/Api/FileUploads/Retrieve/Request/IRetrieveFileUploadQueryParameters.cs	Defined interface for file upload ID path parameter
Src/Notion.Client/Api/FileUploads/Retrieve/FileUploadsClient.cs	Implemented RetrieveAsync method with validation and API call
Src/Notion.Client/Api/FileUploads/IFileUploadsClient.cs	Added RetrieveAsync method signature to interface
Src/Notion.Client/Api/ApiEndpoints.cs	Added Retrieve endpoint URL builder

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