fix(csharp-generichost): honor OR security alternatives for API key auth

[madhus1218]

Fixes #24138.

What changed

This updates the C# generichost generator so OR security requirements no longer apply every auth scheme from every alternative.

For a spec like:

security:
  - apiKeyHeader: []
  - apiKeyQuery: []

---

Summary by cubic

Fixes API key auth handling in the C# generichost generator for OR security. Only the first security alternative is applied, preventing mixed header+query auth.

• Bug Fixes
  • Select the first SecurityRequirement from operation or global security and store its scheme names.
  • Filter op.authMethods to only those schemes for generichost during post-processing.
  • Added a test and example spec to verify only the header API key is used, not the query key.

^{Written for commit 64e49f5. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}