Remove redudant session_id from document path

src/va_agent/session.py

@@ -42,8 +42,9 @@ class FirestoreSessionService(BaseSessionService):
         adk_user_states/{app_name}__{user_id}
             → user-scoped state key/values
 
-        adk_sessions/{app_name}__{user_id}__{session_id}
+        adk_sessions/{app_name}__{user_id}
             → {app_name, user_id, session_id, state: {…}, last_update_time}
+            → Single continuous session per user (session_id is ignored)
             └─ events/{event_id}  →  serialised Event
     """
 
@@ -95,8 +96,9 @@ class FirestoreSessionService(BaseSessionService):
         )
 
     def _session_ref(self, app_name: str, user_id: str, session_id: str) -> Any:
+        # Single continuous session per user: use only user_id, ignore session_id
         return self._db.collection(f"{self._prefix}_sessions").document(
-            f"{app_name}__{user_id}__{session_id}"
+            f"{app_name}__{user_id}"
         )
 
     def _events_col(self, app_name: str, user_id: str, session_id: str) -> Any: