Rename TModel to _TModel to fix broken doc links

Agent-Logs-Url: https://github.com/microsoft/Agent365-python/sessions/dbf4ac27-2d8a-4a26-bc1d-bdd20a8e264c

Co-authored-by: JimDaly <6353736+JimDaly@users.noreply.github.com>

Author: Copilot

libraries/microsoft-agents-a365-notifications/microsoft_agents_a365/notifications/models/agent_notification_activity.py

@@ -7,7 +7,7 @@
 from .email_reference import EmailReference
 from .wpx_comment import WpxComment
 
-TModel = TypeVar("TModel")
+_TModel = TypeVar("_TModel")
 
 
 class AgentNotificationActivity:
@@ -148,7 +148,7 @@ def notification_type(self) -> Optional[NotificationTypes]:
         return self._notification_type
 
     # Generic escape hatch
-    def as_model(self, model: Type[TModel]) -> Optional[TModel]:
+    def as_model(self, model: Type[_TModel]) -> Optional[_TModel]:
         """Parse the activity value as a custom model type.
 
         This method provides a generic way to validate and parse the activity's value