- Run all local engines in parallel using goroutines + sync.WaitGroup
- Individual engine failures are captured as unresponsive_engines entries
instead of aborting the entire search request
- Context cancellation is respected: cancelled engines report as unresponsive
- Upstream proxy failure is also gracefully handled (single unresponsive entry)
- Extract unresponsiveResponse() and emptyResponse() helpers for consistency
- Add comprehensive tests:
- ConcurrentEngines: verifies parallelism (2x100ms engines complete in ~100ms)
- GracefulDegradation: one engine fails, one succeeds, both represented
- AllEnginesFail: no error returned, all engines in unresponsive_engines
- ContextCancellation: engine respects context timeout, reports unresponsive
