Implement agent busy state

[Yahiewi]

This PR solves the issue described here by adding a _busy and _busyChanged attributes to the AgentManager which prevent sending messages and generating a response if the agent is busy.

This screencast shows what happens when we try to send a prompt while the agent is busy:
https://github.com/user-attachments/assets/b3fb6c9d-c7a3-4c01-8e71-467335ef610c

[jtpio]

Thanks @Yahiewi for looking into this!

which prevent sending messages and generating a response if the agent is busy.

Other agents allow sending messages even when already busy with other tasks, which ends up adding messages to a queue that will be processed by the agent at a later time (after a turn or at the end of all turns for instance).

Wondering if we should maybe move towards implementing something like this instead directly? Although we currently may be lacking some UI elements or extension point for such "pending" messages.

[Yahiewi]

Yes I actually agree that being able to send a message but have it pending is a better user experience than simply blocking the user from sending anything.
I thought since this was easier to implement, I would start with it to fix the error, but I'll just extend this PR to add a queue feature.

[jtpio]

Maybe this "queue" component could be a good candidate for https://github.com/brichet/jupyter-chat-components