Python: Fix Postgres filter predicates treated as string literals

[tejasai97]

Summary

• Fixes Python postgres connector #13595: _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-string quoting
• 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 in _construct_vector_query

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

• Added 8 new filter tests; all 28 tests pass

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines
• The pre-submission formatting script raises no violations (ruff check and ruff format)
• All unit tests pass
• New unit tests added for the changes

[tejasai97]

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