Add SQL Server dialect fixtures for read-only validation

[Victoria824]

Why

SQL safety rules should behave predictably across common enterprise dialects. SQL Server syntax is not represented by a focused regression fixture today.

Acceptance criteria

• Add safe SELECT fixtures using brackets, TOP, and schema-qualified tables
• Add blocked fixtures for writes, multi-statement batches, and unsafe external access
• Verify table and column grounding for bracket-quoted identifiers
• Keep all cases deterministic and runnable without SQL Server
• Document any intentionally unsupported syntax

Helpful starting points

• src/queryassure/validators.py
• evals/chaos.yml
• tests_py/test_security.py