When a background process with notify_on_complete=True finishes, the
gateway injects a synthetic MessageEvent to notify the session. This
event was constructed without user_id, causing _is_user_authorized()
to reject it and — for DM-origin sessions — trigger the pairing flow,
sending "Hi~ I don't recognize you yet!" with a pairing code to the
chat owner.
Add an `internal` flag to MessageEvent that bypasses authorization
checks for system-generated synthetic events. Only the process watcher
sets this flag; no external/adapter code path can produce it.
Includes 4 regression tests covering the fix and the normal pairing path.
1d8d4f28ae