test: cover AI model deserialization patterns

[ynachiket]

Summary

• add metadata coverage for the existing AI203 and AI204 model deserialization rules
• add focused detection tests for keras.models.load_model(...) and joblib.load(...)
• add comment-line exclusion checks so the regex rules do not flag commented examples

Closes #16

Validation

• /tmp/pyspector-pdlc047-venv/bin/python -m pytest tests/unit/test_ai_rules.py -q -> 4 passed, 6 skipped (scanner-path tests skipped because the local runner lacks the PySpector Rust core)
• /tmp/pyspector-pdlc047-venv/bin/python -m py_compile tests/unit/test_ai_rules.py
• git diff --check