[codex] Scope AI config failures to analysis endpoint

[amodelandme]

Summary

• Add an unavailable IAiFlagAnalyzer implementation for missing Azure OpenAI configuration.
• Register the unavailable analyzer when AzureOpenAI:Endpoint is blank or missing instead of throwing during startup.
• Add integration coverage proving non-AI endpoints still respond while /api/flags/health returns the documented 503 ProblemDetails response.
• Add the approved PRD for AI startup resilience.

Why

The Phase 1 / 1.5 architecture review found that missing Azure OpenAI configuration could crash the entire API at startup. AI flag health analysis is optional and should fail at the endpoint boundary, not take down unrelated CRUD or evaluation endpoints.

Validation

• dotnet test Banderas.Tests.Integration/Banderas.Tests.Integration.csproj --no-restore
• dotnet test Banderas.Tests/Banderas.Tests.csproj --no-restore

Notes

• Left the pre-existing local .gitignore change unstaged and out of this PR.
• Infrastructure format verification still reports an unrelated existing whitespace issue in Banderas.Infrastructure/Telemetry/NullTelemetryService.cs.