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- transportsMu (sync.RWMutex): guard getOrCreateTransport reads/writes;
double-checked locking pattern; safe for dynamic backends
- applyWarmup: replace time.Sleep with select{time.After, ctx.Done}
so cancelled warmup requests release the goroutine immediately
- Retry-After: 5 header on queue-full 503 (client backoff hint)
- OTel trace_id skipped: collector in tracing ns, not monitoring ns