refactor(BA-5712): address slice D review feedback

- repositories/session/creators.py: docstring pointed to a non-existent
  ``owner_id`` attribute on SessionRow; clarify that the scope_id
  comes from ``user_uuid`` (the owner's UUID).
- repositories/session/db_source/db_source.py: drop the redundant
  ``session_row.user_uuid is not None`` guard (the column is
  non-nullable); update the ``_find_dependent_sessions`` docstring to
  reference ``owner_id`` instead of the removed ``access_key`` param.

Author: jopemachine

src/ai/backend/manager/repositories/session/creators.py

@@ -17,8 +17,8 @@ class SessionRowCreatorSpec(CreatorSpec[SessionRow]):
     This spec is designed for retrofitting existing code that already builds
     SessionRow instances. It simply returns the provided row in build_row().
 
-    For scope information needed by RBACEntityCreator, use the row's owner_id
-    field as the scope_id with ScopeType.USER.
+    For scope information needed by RBACEntityCreator, use the row's user_uuid
+    field (the owner's UUID) as the scope_id with ScopeType.USER.
     """
 
     row: SessionRow

src/ai/backend/manager/repositories/session/db_source/db_source.py

@@ -305,7 +305,9 @@ async def modify_session(
             if session_row is None:
                 raise SessionNotFound(f"Session not found (id:{session_id})")
 
-            if session_name and session_row.user_uuid is not None:
+            if session_name:
+                # ``SessionRow.user_uuid`` is non-nullable, so we only need to
+                # gate on whether a new name was provided.
                 # Check the owner of the target session has any session with the same name
                 try:
                     sess = await SessionRow.get_session(
@@ -379,7 +381,7 @@ async def _find_dependent_sessions(
 
         :param db_sess: Database session
         :param root_session_name_or_id: Root session name or ID
-        :param access_key: Access key of the session owner
+        :param owner_id: UUID of the session owner
         :param allow_stale: Whether to allow stale sessions
         :return: Tuple of (root_session_id, set of dependent session IDs)
         """