server: release the session lock when a client disconnects mid-stream

[jethac]

Problem

opus_reader.read_pcm() returns None once the client's opus stream ends — which happens whenever a client goes away mid-session rather than closing cleanly. opus_loop then evaluates pcm.shape[-1] on None and raises inside the async with self.lock block, so the task dies while still holding the session lock.

What makes this awkward to diagnose is that the server does not crash. It keeps serving HTTP and accepting websocket upgrades, but every subsequent /api/chat blocks forever waiting for a lock that will never be released. Only a restart clears it.

Task exception was never retrieved
future: <Task finished coro=<ServerState.handle_chat.<locals>.opus_loop() ...>
    exception=AttributeError("'NoneType' object has no attribute 'shape'")>
  File ".../moshi/server.py", line 212, in opus_loop
    if pcm.shape[-1] == 0:
       ^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'shape'

Reproduction

1. Connect a client and start streaming audio.
2. Kill it without closing the socket (drop the process, pull the network).
3. Connect again — the new session hangs indefinitely.

Hit this in practice with a robot client on flaky wifi; it also shows up any time a browser tab is closed mid-conversation.

Fix

Treat None as the stream-ended signal it is and return, which unwinds the context manager and frees the lock for the next session. Four lines, no behaviour change on the normal path.

Independent of any hardware or quantization work — applies to main as-is.