Python: Fix Postgres filter predicates treated as string literals

[tejasai97]

Summary

• Fixes Python postgres connector #13595 where _lambda_parser in the Postgres connector returned plain Python strings for SQL WHERE clauses, which psycopg's sql.SQL.format() auto-wrapped as sql.Literal (quoted string literals) instead of embedding as SQL expressions
• Changed _lambda_parser to return sql.Composable objects (sql.Identifier, sql.Literal, sql.SQL) so predicates are correctly embedded as SQL fragments
• Added 8 unit tests covering equality, numeric comparison, AND/OR, IN, NOT, multiple filters, and chain comparisons

Motivation and Context

When using lambda filters with the Postgres vector search (e.g., filter=lambda x: x.content_type == "course"), the generated SQL was malformed:

-- Before (broken): predicate quoted as a string literal
WHERE '"content_type" = ''course'''

-- After (fixed): predicate embedded as a SQL expression
WHERE "content_type" = 'course'

Description

python/semantic_kernel/connectors/postgres.py:

• _lambda_parser now returns sql.Composable instead of str
• Column names use sql.Identifier() instead of manual f'"{name}"'
• Constants use sql.Literal() instead of manual string escaping
• Booleans/NULL use sql.SQL("TRUE") / sql.SQL("NULL")
• All operators compose via sql.SQL("{} op {}").format(left, right)
• Fixed missing leading space in WHERE clause

python/tests/unit/connectors/memory/test_postgres_store.py:

• Added 8 new filter tests; all 28 tests pass

Test plan

• All 28 existing + new unit tests pass (pytest tests/unit/connectors/memory/test_postgres_store.py)
• ruff check passes with no errors
• ruff format produces no changes
• CI pipeline passes

🤖 Generated with Claude Code

[tejasai97]

Closing this PR - submitted in error.