test: cover OpenAPI parser malformed-doc, inline response, and $ref body paths

[Theauxm]

Summary

Adds three OpenAPI parser tests that hit branches with no prior coverage:

• Malformed/empty document — exercises the diagnostic-error throw path
• Inline response object schema — BuildOutputType inline-properties branch (and the Unit-fallback branches for no-content and non-JSON responses)
• $ref request body — BuildInputType reference-resolving branch

Two small fixtures added under tests/Trax.Cli.Tests/Fixtures/Schemas/.

Local line coverage moved from ~89% to ~90% on Trax.Cli.

Test plan

• dotnet test tests/Trax.Cli.Tests — all passing (3 new)
• dotnet csharpier check . — clean

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

ℹ️ You can also turn on project coverage checks and project coverage reporting on Pull Request comment

Thanks for integrating Codecov - We've got you covered ☂️