added mention support in stream_message()

[Abigayle-Mercer]

This PR enables persona-to-persona mentions in streaming responses by leveraging the find_mentions parameter added to YChat.update_message() in the upstream jupyter-chat library.

Changes:

• Added trigger_actions=[] to update_message() calls during streaming to defer mention extraction
• Added final update_message(..., trigger_actions=[find_mentions_callback]) call after streaming completes to trigger mention extraction and persona notifications
• Imported find_mentions_callback from jupyterlab_chat.utils
• Updated comments to document mention detection behavior

How it works:

1. During streaming: Each chunk calls update_message(..., append=True, trigger_actions=[]) - no callbacks are executed, mentions are NOT extracted

2. When complete: Final call to update_message(..., trigger_actions=[find_mentions_callback]) executes the mention extraction callback and notifies mentioned personas

This ensures mentioned personas are only notified once when the complete message is ready, rather than on every incremental update.

Dependencies:

Requires the callback-based trigger_actions parameter in YChat.update_message() and YChat.add_message() from jupyterlab/jupyter-chat (PR #302).