[Bug]: channel reply stream can block forever after Redis Pub/Sub disconnect

## Bug description

The Redis-backed channel reply stream can remain blocked forever when its
Pub/Sub connection is reset after the subscription has become ready.

RedisMessageBus.subscribe() lets the connection exception escape after its
Pub/Sub feeder is closed. open_reply_stream() only handles cancellation of
that feeder, while _read() waits indefinitely on the local queue. The
session event has already been appended to the replay log by
publish_session_event(), but the reader never reconnects or replays the
event.

## Reproduction

Using the production RedisMessageBus and open_reply_stream() with a
fakeredis-backed client, inject the same
redis.exceptions.ConnectionError that a reset Redis connection raises after
the first subscription is ready. Then publish a terminal ReplyEndEvent via
publish_session_event().

Observed:

STREAM_OUTCOME=TimeoutError: reader remained blocked
PUBSUB_SUBSCRIPTIONS=1

The event is present in the replay log, but the reply reader remains pending
because the subscription task has exited and no new wake-up reaches its local
queue.

## Expected behavior

A channel reply stream should recover from a transient Redis Pub/Sub
disconnect, resubscribe, and replay events appended while the subscription
was unavailable. It should deliver the terminal event and finish normally.

## Impact

A Redis restart, connection reset, or brief network interruption can leave a
channel reply delivery task waiting indefinitely. The user may never receive
the final reply even though the event was durably written to the session
replay log.

## Related work

This is distinct from #2123 and #2124, which address the replay/live
subscription handoff and SSE replay window. Those changes do not recover a
reply stream after its Pub/Sub feeder task terminates.

I would like to take this fix myself. If this scope and direction look right, please explicitly confirm that I may implement it, and I will prepare a focused test-first PR. I will not start the implementation before maintainer approval.