test: add Xquik OpenAPI fixture

[kriptoburak]

Summary

• Add a compact OpenAPI 3.1 fixture based on the public Xquik contract.
• Cover API-key header auth, a nested API path, schema refs, and disclosure rendering.
• Fix the repository installation path and documented build output location.

Validation

• go test ./...
• go vet ./...
• make test
• make lint
• go mod tidy with no dependency diff

[kriptoburak]

Rebased this contribution on the current main branch as 1 signed commit.

The repair keeps the Xquik OpenAPI 3.1 fixture and adds an independent documentation fix: the go install module path now matches this repository and the build output location is accurate.

Validation passed: go test ./..., go vet ./..., make test, make lint, and go mod tidy with no dependency diff.