test: add Xquik OpenAPI 3.1 fixture

[kriptoburak]

Summary

• Add a compact Xquik OpenAPI 3.1 validation fixture.
• Cover the public tweet search operation, response schema refs, and API key header security scheme.

Validation

• Installed dependencies without writing a lockfile because this repo has no package lock.
• Focused Mocha run for the Xquik fixture passed with 1 test.

[kriptoburak]

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