test: cover OpenAPI parser collision and empty-ref edge cases

[Theauxm]

Summary

• Adds a fixture and four targeted parser tests for OpenAPI edges that were not previously hit:
  • Synthesized operation name colliding with a component schema (verb-prefix path)
  • Operation-id collision where path-param disambiguation runs out and falls back to a numeric suffix
  • Empty component type referenced directly and inside a list (rewriting to object / List<object>)
  • Inline-promoted object name collision producing a numeric suffix via EnsureUniqueName
• Lifts local Cobertura line coverage from 95.99% to 96.39% (97.30% excluding generated regex files).

Test plan

• dotnet test (293 passed, 0 failed)
• dotnet build zero new warnings
• dotnet csharpier format . clean
• Cobertura report shows the targeted lines now covered (233, 692, 637/647, 708-713)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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