support proactive-voice discussion for sessions, with a locking bus to guarantee sequential chats

[kwannoel]

Summary

Add support for proactive-voice discussion capabilities for sessions, where the system can initiate conversations with the user rather than only responding. Implement a locking bus mechanism to guarantee that chats within a session are processed sequentially, preventing race conditions from concurrent messages.

Details

Currently, sessions only support reactive interactions — the user sends a message and gets a response. This feature introduces proactive-voice discussion, allowing the system to surface observations, suggestions, or status updates without being prompted.

Key requirements:

• Proactive messaging: Sessions can emit discussion messages triggered by events (e.g., task completion, errors, idle detection)
• Locking bus: A sequential message bus that acquires a lock before processing each chat message, ensuring strict ordering and preventing interleaving of proactive and user-initiated messages
• Lock lifecycle: Lock is acquired before sending, held through response completion, then released for the next queued message
• Queue management: When the bus is locked, incoming messages (both proactive and user-driven) are queued and processed FIFO
• Backpressure: Configurable queue depth to prevent unbounded accumulation of proactive messages

This ensures conversation coherence regardless of whether messages originate from the user or the system.