test: add Xquik OpenAPI 3.1 fixture

[kriptoburak]

Summary

• Add a compact Xquik OpenAPI 3.1 fixture to the document loader tests.
• Cover the public tweet search path, response schema references, and API key header security scheme.

Validation

• Focused Go test for TestLoadDocument_XquikOpenAPI31 passed.
• Full package sweep was not used because the local machine had only 1.3 GiB free and the broad Go build hit temp-space limits outside this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.75%. Comparing base (d4c45ce) to head (fd2b6be).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #599   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files         281      281           
  Lines       34061    34096   +35     
=======================================
+ Hits        33977    34012   +35     
  Misses         51       51           
  Partials       33       33           

Flag	Coverage Δ
unittests	99.75% <ø> (+<0.01%)	⬆️

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[daveshanley]

I don't mean to be rude, but what is this for?

[kriptoburak]

Thanks for taking a look. You are right to question this: as a standalone branded fixture, it does not add enough value to libopenapi's test suite. I am closing it rather than asking you to carry a low-signal example.