Python: Bug: SqlServerCollection renders x.tenant == None as [tenant] = ? with the string 'None' instead of IS NULL #14683

Describe the bug

SqlServerCollection renders None in a search filter as the string literal 'None', so a filter on a nullable field can never match SQL NULL:

'lambda x: x.tenant == None'  ->  [tenant] = ?          params ['None']
'lambda x: x.tenant != None'  ->  [tenant] <> ?         params ['None']
'lambda x: x.tenant is None'  ->  NotImplementedError: Unsupported operator: <class 'ast.Is'>

The full generated query for a default_dynamic_filter_function filter with a None argument is:

SELECT [id], [tenant], [active], VECTOR_DISTANCE('cosine', [embedding], CAST(? AS VECTOR(3))) as _vector_distance_value
 FROM [dbo].[filter_records]
 WHERE [tenant] = ?
ORDER BY _vector_distance_value ASC
OFFSET 0 ROWS FETCH NEXT 3 ROWS ONLY;
parameters: ['[1.0, 0.0, 0.0]', 'None']

Root cause: _lambda_parser binds every value with command.add_parameter(str(value)) (python/semantic_kernel/connectors/sql_server.py:659-664) and the Eq/NotEq handlers (:611-616) have no NULL branch, so None becomes the varchar 'None'. Booleans are also bound as 'True'/'False' here even though _cast_value (:850) already maps them to 1/0.

To Reproduce

A self-contained script (no database needed) builds SqlServerCollection with a nullable tenant field and asks it to parse the filters above; it prints the SQL and parameter values shown above. Run from python/:

.venv/Scripts/python.exe -I repro_sqlserver_null_filter.py

The same filter path is what default_dynamic_filter_function produces when a search argument is None, so ordinary searches with an omitted nullable argument are affected.

Expected behavior

• x.tenant == None / x.tenant is None → [tenant] IS NULL
• x.tenant != None → [tenant] IS NOT NULL
• x.active == True → parameter 1 (matching _cast_value)

The Postgres collection already does exactly this (fixed in #14497, covered by test_null_filter_parser); SQL Server was missed.

Platform

• Language: Python
• Source: main branch at 4bb64ee (2026-10-28), semantic-kernel 1.46.0
• AI model: n/a (no model call in the reproduction)
• OS: Windows 11

Additional context

• Searched open and closed issues/PRs for SqlServerCollection filters and SQL Server NULL handling: no duplicate; no open PR touches sql_server.py.
• Suggested fix: add an ast.Is/ast.IsNot branch and NULL handling in the Eq/NotEq visitors (compare with the Postgres parser), and bind booleans via _cast_value.

Reported with AI assistance (analysis and reproduction script); the reporter ran the reproduction and verified the output above.