Hanging up as a SIP participant message doesn't get committed to the chat_ctx

[martin-purplefish]

We're using the on_agent_message_committed to log the transcript to our backend.

The behavior we've noticed is that when the user hangs up, while the agent stops speaking, we don't get a final on_agent_message_committed - so that final cut off message doesn't appear in our own transcript.

To reproduce:

1. Add callback handlers for on_agent_message_committed and on_agent_message_interrupted that log.
2. Call the agent.
3. Hang up the phone half way through.

The audio (at least on egress) will stop, but no callbacks will happen.

[martin-purplefish]

#1179 would be a fix.

[martin-purplefish]

Also happens for if the candidate is speaking when they hang up (rare in practice) - their last message isn't committed.