test: preserve anonymous security alternatives

[kriptoburak]

Summary

• Add regression coverage for OpenAPI 3.1 metadata with an anonymous security alternative and an API key alternative.
• Use a compact Xquik-style security array to ensure APISpec preserves both entries.

Validation

• Python 3.11: pytest tests/test_core.py -q, 155 passed.
• Diff whitespace check passed.

[kriptoburak]

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