Bot message streaming API

## Summary
Implement a bot message streaming API so integrations can progressively update responses instead of waiting for one final message.

## Scope
- Support incremental bot output for long-running or agentic responses.
- Define how partial bot output appears in the transcript while preserving message history clarity.
- Support completion, cancellation, timeout, and error states.
- Keep permissions and rate limits aligned with the rest of the message system.

## Acceptance Criteria
- [ ] Bots can start a response and stream visible updates into the same message lifecycle.
- [ ] Users can distinguish in-progress bot output from completed output.
- [ ] Cancellation and failure states are explicit and do not leave broken transcript artifacts.
- [ ] Streaming behavior works through the main realtime path rather than a disconnected side channel.

## Operational Notes
- This feature should be designed for real server-local automation and agent use, not only toy demos.
- Streaming semantics must remain understandable in logs, moderation tools, and future exports.