test: cover optional security alternatives

[kriptoburak]

Summary

Adds a regression case for operation-level security alternatives that allow either an API key or anonymous access.

The existing security override fixture already covers default, override, and removal behavior. This adds the OpenAPI pattern { api_key: [] }, {} for both 3.0 and 3.1 fixtures and runs the existing tests across both unmarshallers.

Validation

• python3 -m py_compile tests/integration/unmarshalling/test_security_override.py
• git diff --check
• python3 -m pytest tests/integration/unmarshalling/test_security_override.py was attempted but could not run because jsonschema_path is not installed in this environment.