fix(gateway): propagate user identity through process watcher pipeline

Background process watchers (notify_on_complete, check_interval) created
synthetic SessionSource objects without user_id/user_name. While the
internal=True bypass (1d8d4f28) prevented false pairing for agent-
generated notifications, the missing identity caused:

- Garbage entries in pairing rate limiters (discord:None, telegram:None)
- 'User None' in approval messages and logs
- No user identity available for future code paths that need it

Additionally, platform messages arriving without from_user (Telegram
service messages, channel forwards, anonymous admin actions) could still
trigger false pairing because they are not internal events.

Fix:
1. Propagate user_id/user_name through the full watcher chain:
   session_context.py → gateway/run.py → terminal_tool.py →
   process_registry.py (including checkpoint persistence/recovery)

2. Add None user_id guard in _handle_message() — silently drop
   non-internal messages with no user identity instead of triggering
   the pairing flow.

Salvaged from PRs #7664 (kagura-agent, ContextVar approach),
#6540 (MestreY0d4-Uninter, tests), and #7709 (guang384, None guard).

Closes #6341, #6485, #7643
Relates to #6516, #7392

tests/tools/test_process_registry.py

@@ -438,6 +438,8 @@ class TestCheckpoint:
             s = _make_session()
             s.watcher_platform = "telegram"
             s.watcher_chat_id = "999"
+            s.watcher_user_id = "u123"
+            s.watcher_user_name = "alice"
             s.watcher_thread_id = "42"
             s.watcher_interval = 60
             registry._running[s.id] = s
@@ -447,6 +449,8 @@ class TestCheckpoint:
             assert len(data) == 1
             assert data[0]["watcher_platform"] == "telegram"
             assert data[0]["watcher_chat_id"] == "999"
+            assert data[0]["watcher_user_id"] == "u123"
+            assert data[0]["watcher_user_name"] == "alice"
             assert data[0]["watcher_thread_id"] == "42"
             assert data[0]["watcher_interval"] == 60
 
@@ -460,6 +464,8 @@ class TestCheckpoint:
             "session_key": "sk1",
             "watcher_platform": "telegram",
             "watcher_chat_id": "123",
+            "watcher_user_id": "u123",
+            "watcher_user_name": "alice",
             "watcher_thread_id": "42",
             "watcher_interval": 60,
         }]))
@@ -471,6 +477,8 @@ class TestCheckpoint:
             assert w["session_id"] == "proc_live"
             assert w["platform"] == "telegram"
             assert w["chat_id"] == "123"
+            assert w["user_id"] == "u123"
+            assert w["user_name"] == "alice"
             assert w["thread_id"] == "42"
             assert w["check_interval"] == 60