Closes https://github.com/zed-industries/zed/issues/47330

This PR moves the queue logic out of the native Thread into the shared
UI layer (thread view) to enable it for external agents. There's a key
difference in behavior, though, between native and external agents:
queued messages in the former will be sent in the next turn boundary,
given we can easily tell this, whereas for the latter, queued messages
will be sent by the end of the generation. We'd need an ACP-level change
to provide exactly the same UX between both types of agents, and I
figured that's better to have _some_ version of the feature for external
agents as opposed to not having it all due to this difference.

Release Notes:

- Agent: Made the message queue feature available for external agents as
well.