bus: replace a redis subscription that stops answering pings

[cosmin-staicu]

The Redis bus reads its subscription with ReceiveMessage, which blocks with no deadline and never pings. go-redis only health-checks a PubSub through its Channel() API, so a subscription socket that silently stops delivering is never noticed. Two ways this happens: the connection drops without a FIN, or Redis stops serving it once the credentials it authenticated with expire (go-redis re-authenticates pooled connections on streaming credential updates, but not PubSub ones). The process keeps publishing while it receives nothing, and RPCs fail with "no response from servers" until a later SUBSCRIBE or UNSUBSCRIBE happens to hit a write error.

The read now uses ReceiveTimeout. A read that sees nothing for the health check interval (30s) sends a PING. If the next interval passes with no reply of any kind, or the subscription returns a Redis error reply such as NOAUTH, the bus opens a new PubSub subscribed to every current channel and closes the old one. A quiet connection that answers its PING is kept.

The replacement and the subscription reconciler are serialized on a new mutex, and the reconciler holds it until currentChannels reflects its change, so a replacement always subscribes exactly the current set.

The tests put a TCP proxy between the subscriber and Redis that can silence the connections it holds without closing them, or inject an error reply, and check that delivery resumes on a new connection within two intervals and that a quiet healthy subscription is kept. A constructor in export_test.go shortens the interval for them, so the public API is unchanged.