FEAT: streaming support in fetchone for varcharmax data type (#219)

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[AB#38110](https://sqlclientdrivers.visualstudio.com/c6d89619-62de-46a0-8b46-70b92a84d85e/_workitems/edit/38110)

[AB#34162](https://sqlclientdrivers.visualstudio.com/c6d89619-62de-46a0-8b46-70b92a84d85e/_workitems/edit/34162)

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### Summary   
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This pull request significantly improves the handling of large object
(LOB) data types (such as large strings and binary data) in the MSSQL
Python driver, especially for fetching and streaming variable-length
data. The changes introduce robust streaming logic for LOB columns,
prevent data truncation, and ensure correct type handling for both
single-row and batch fetches. Additionally, the code now detects LOB
columns and automatically switches to per-row streaming when necessary,
improving reliability and correctness for large datasets.

**LOB Streaming and Fetching Improvements:**

* Introduced the `FetchLobColumnData` function in `ddbc_bindings.cpp` to
stream LOB data (CHAR, WCHAR, and BINARY types) in chunks, correctly
handling nulls, null-terminators, and platform-specific encoding. This
prevents truncation and errors when fetching large columns.
* Updated `SQLGetData_wrap` to use streaming for LOB columns or when
data length is unknown/too large, for both narrow and wide character
types, as well as binary data. This ensures correct retrieval of all
data regardless of size.
[[1]](diffhunk://#diff-dde2297345718ec449a14e7dff91b7bb2342b008ecc071f562233646d71144a1L1749-L1771)
[[2]](diffhunk://#diff-dde2297345718ec449a14e7dff91b7bb2342b008ecc071f562233646d71144a1L1782-L1800)
[[3]](diffhunk://#diff-dde2297345718ec449a14e7dff91b7bb2342b008ecc071f562233646d71144a1R1916-R1967)

**Batch Fetch Logic Enhancements:**

* Modified `FetchBatchData` to detect LOB columns and use streaming
fetch for those columns, avoiding exceptions and ensuring all data is
retrieved for large columns in batch operations.
[[1]](diffhunk://#diff-dde2297345718ec449a14e7dff91b7bb2342b008ecc071f562233646d71144a1L2319-R2415)
[[2]](diffhunk://#diff-dde2297345718ec449a14e7dff91b7bb2342b008ecc071f562233646d71144a1L2379-R2487)
[[3]](diffhunk://#diff-dde2297345718ec449a14e7dff91b7bb2342b008ecc071f562233646d71144a1R2499-R2501)
[[4]](diffhunk://#diff-dde2297345718ec449a14e7dff91b7bb2342b008ecc071f562233646d71144a1L2424-R2515)
[[5]](diffhunk://#diff-dde2297345718ec449a14e7dff91b7bb2342b008ecc071f562233646d71144a1L2516-R2609)
* Updated `FetchMany_wrap` to pre-scan columns for LOB types and, if any
are found, fall back to row-by-row streaming fetch for those rows;
otherwise, it proceeds with standard batch fetching.

**Type Mapping and Constants:**

* Adjusted `_map_sql_type` in `cursor.py` to map long string types to
`SQL_WVARCHAR`/`SQL_VARCHAR` with length 0 for streaming, aligning with
the new LOB streaming logic.
* Defined `SQL_MAX_LOB_SIZE` (8000) as the threshold for LOB streaming,
centralizing the logic for when to treat columns as LOBs.

These changes collectively make LOB handling more robust, reduce the
risk of data truncation, and improve compatibility across platforms.
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Author: gargsaumya

mssql_python/cursor.py

@@ -373,16 +373,16 @@ def _map_sql_type(self, param, parameters_list, i):
             if utf16_len > MAX_INLINE_CHAR:  # Long strings -> DAE
                 if is_unicode:
                     return (
-                        ddbc_sql_const.SQL_WLONGVARCHAR.value,
+                        ddbc_sql_const.SQL_WVARCHAR.value,
                         ddbc_sql_const.SQL_C_WCHAR.value,
-                        utf16_len,
+                        0,
                         0,
                         True,
                     )
                 return (
-                    ddbc_sql_const.SQL_LONGVARCHAR.value,
+                    ddbc_sql_const.SQL_VARCHAR.value,
                     ddbc_sql_const.SQL_C_CHAR.value,
-                    len(param),
+                    0,
                     0,
                     True,
                 )