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.