fix(api): transparently proxy responses compact endpoint

[GcsSloop]

Summary

• transparently proxy /responses/compact and /v1/responses/compact instead of returning 404
• keep other unsupported synthetic responses subpaths returning 404
• add regression coverage for compact passthrough behavior

Verification

• cd backend && go test ./internal/api -run 'TestResponsesHandlerThinModePassesThroughCompactSubpath|TestResponsesHandlerThinModeDisablesSyntheticEndpoints|TestResponsesHandlerThinModeThirdPartyResponsesPassthrough|TestResponsesHandlerThinModePassesThroughPreviousResponseID'
• cd backend && go test ./internal/api ./internal/bootstrap ./internal/providers/...