Add Xquik OpenAPI 3.1 parse fixture

[kriptoburak]

Summary

• Add a compact Xquik OpenAPI 3.1 example fixture.
• Assert gnostic parses the fixture title and Xquik search path.
• Escape JSON object keys so quotes, backslashes, and newlines produce valid JSON.
• Add a regression test for escaped mapping keys.
• Replace deprecated ioutil.ReadFile calls in the touched parser tests.

Validation

• go test ./jsonwriter ./openapiv3
• go vet ./jsonwriter ./openapiv3
• git diff --check

The JSON key fix is independent of the Xquik fixture and improves all jsonwriter.Marshal callers.

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[kriptoburak]

I added a repository-wide JSON writer repair alongside the Xquik fixture. Mapping keys now use the same escaping path as scalar strings, with coverage for quotes, backslashes, and newlines. I also replaced deprecated ioutil.ReadFile usage in the touched parser tests.

Focused tests and vet pass locally. The branch is current with main; the CLA check is now green.