OnStreamResponse and OnStreamDeltaResponse in internal/xds/cache/snapshotcache.go read from s.streamIDNodeInfo without holding the mutex lock.

The comment says "No mutex lock required here because no writing to the cache" — but this is incorrect. Other callbacks (OnStreamOpen, OnStreamClosed, OnStreamRequest, OnDeltaStreamOpen, OnDeltaStreamClosed) all write to this same map while holding s.mu. In Go, concurrent map read and write is a data race that causes fatal error: concurrent map read and map write.

Affected code:

• OnStreamResponse (line ~277): reads s.streamIDNodeInfo[streamID] without lock
• OnStreamDeltaResponse (line ~396): same pattern

Impact: Under concurrent xDS traffic (multiple Envoy proxies connecting/disconnecting), this can crash the control plane with a runtime panic.

Fix: Acquire s.mu.RLock() before reading the map in both functions.