The test_non_internal_event_without_user_triggers_pairing test relied on
no Discord auth env vars being set, but gateway/run.py loads dotenv at
module level. In environments with DISCORD_ALLOW_ALL_USERS=True in .env,
the auth check passed instead of triggering the pairing flow.
Clear DISCORD_ALLOW_ALL_USERS, DISCORD_ALLOWED_USERS, GATEWAY_ALLOW_ALL_USERS,
and GATEWAY_ALLOWED_USERS via monkeypatch to ensure test isolation.
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.
