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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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required. -->
This pull request improves NVARCHAR data handling in the SQL Server
Python bindings and adds comprehensive tests for NVARCHAR(MAX)
scenarios. The main changes include switching to streaming for large
NVARCHAR values, optimizing direct fetch for smaller values, and adding
tests for edge cases and boundaries to ensure correctness.

**NVARCHAR data handling improvements:**

* Updated the logic in `ddbc_bindings.cpp` to use streaming for large
NVARCHAR/NCHAR columns (over 4000 characters or unknown size) and direct
fetch for smaller values, optimizing performance and reliability.
* Refactored data conversion for NVARCHAR fetches, using `std::wstring`
for conversion and simplifying platform-specific handling for both
macOS/Linux and Windows.
* Improved handling of empty strings and NULLs for NVARCHAR columns,
ensuring correct Python types are returned and logging is more
descriptive.

**Testing enhancements:**

* Added new tests in `test_004_cursor.py` for NVARCHAR(MAX) covering
short strings, boundary conditions (4000 chars), streaming (4100+
chars), large values (100,000 chars), empty strings, NULLs, and
transaction rollback scenarios to verify correct behavior across all
edge cases.

**VARCHAR/CHAR fetch improvements:**

* Improved direct fetch logic for small VARCHAR/CHAR columns and fixed
string conversion to use the actual data length, preventing potential
issues with null-termination and buffer size.
[[1]](diffhunk://#diff-dde2297345718ec449a14e7dff91b7bb2342b008ecc071f562233646d71144a1R1825-R1830)
[[2]](diffhunk://#diff-dde2297345718ec449a14e7dff91b7bb2342b008ecc071f562233646d71144a1L1841-L1850)

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