When session A (e.g. websocket) uses the `message` tool to send to
channel B (e.g. feishu), the outbound message is delivered to the user
but was never recorded in session B's history. This caused session B to
lose context when the user replied on that channel.
Add `_persist_cross_channel_calls()` to detect cross-channel `message`
tool calls during `_save_turn()` and append a lightweight assistant
entry (with `_cross_channel: True` marker) to the target session.
Track text-only user messages that were flushed before the turn loop completes, then materialize an interrupted assistant placeholder on the next request so session history stays legal and later turns do not skip their own assistant reply.
Made-with: Cursor
Use session.add_message for the pre-turn user-message flush and add focused regression tests for crash-time persistence and duplicate-free successful saves.
Made-with: Cursor
