fix(discord): propagate role_authorized flag so DISCORD_ALLOWED_ROLES works end-to-end

DISCORD_ALLOWED_ROLES was checked by the Discord adapter (_is_allowed_user)
but gateway._is_user_authorized only read DISCORD_ALLOWED_USERS, so
role-authorized users were rejected with "Unauthorized user" at the
gateway layer despite passing the adapter gate.

- Add role_authorized: bool = False to SessionSource
- Add role_authorized param to build_source (base.py)
- Compute _role_authorized in on_message when user passes via role not user ID
- Thread _role_authorized through _handle_message -> build_source
- Check source.role_authorized early in _is_user_authorized (run.py)

Fixes #33952

gateway/session.py

@@ -91,6 +91,7 @@ class SessionSource:
     guild_id: Optional[str] = None  # Discord guild / Slack workspace / Matrix server scope
     parent_chat_id: Optional[str] = None  # Parent channel when chat_id refers to a thread
     message_id: Optional[str] = None  # ID of the triggering message (for pin/reply/react)
+    role_authorized: bool = False  # True when adapter granted access via role (not user ID)
     
     @property
     def description(self) -> str: