WIP: fix(pubsub): remove actors that are no longer processing messages

[PhongChuong]

The Dispatcher now removes batch_actors entries that are no longer processing messages.

A batch actor will close its channel and begin shutdown when it detects that it has no pending messages. Any remaining messages in the channel will be handled by the run loop.

When an actor shutdown, the Dispatcher joins its task and removes the entry from batch_actors. Since sequential actors requires batches to complete before the next batch can be sent, we keep a buffer of messages that arrives after the channel is closed. This buffer is passed onto the next batch actor is needed.

This PR updates 2 tests to reflect the new behavior:

1. client::batch_sends_on_byte_threshold: don't check the number of times the rpc is performed since the actor shutdown can change how batching is performed.
2. actor::sequential_actor_flush: don't check empty flush since it causes the actor to shutdown. Plus, the case is already covered by the test client::flush_with_no_messages_is_noop

Fixes #4012

[codecov]

Codecov Report

❌ Patch coverage is 70.83333% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.46%. Comparing base (9fe2fd7) to head (108b38d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pubsub/src/publisher/actor.rs	70.83%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4897      +/-   ##
==========================================
- Coverage   94.55%   94.46%   -0.10%     
==========================================
  Files         213      213              
  Lines        8378     8384       +6     
==========================================
- Hits         7922     7920       -2     
- Misses        456      464       +8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.