fix: allow anonymous security alternatives

[kriptoburak]

Summary

• Treat an empty OpenAPI security requirement object as an anonymous alternative.
• Keep endpoints that allow API key auth or anonymous access callable with Client as well as AuthenticatedClient.
• Add parser coverage for anonymous-only, API-key-or-anonymous, and API-key-only operations.

Validation

• uv run --python 3.13 --with mypy --with pytest --with pytest-mock --with pydantic --with attrs --with httpx --with jinja2 --with typer --with ruamel.yaml --with python-multipart --with syrupy python -m pytest tests/test_parser/test_openapi.py -q
• uv run --python 3.13 ruff check openapi_python_client/parser/openapi.py tests/test_parser/test_openapi.py
• uv run --python 3.13 ruff format --check openapi_python_client/parser/openapi.py tests/test_parser/test_openapi.py