fix(gateway): prevent background process notifications from triggering false pairing requests

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.

gateway/platforms/base.py

@@ -407,6 +407,10 @@ class MessageEvent:
     # Auto-loaded skill for topic/channel bindings (e.g., Telegram DM Topics)
     auto_skill: Optional[str] = None
     
+    # Internal flag — set for synthetic events (e.g. background process
+    # completion notifications) that must bypass user authorization checks.
+    internal: bool = False
+
     # Timestamps
     timestamp: datetime = field(default_factory=datetime.now)