refactor: Simplify and update comments

src/ai/backend/manager/models/utils.py

@@ -422,8 +422,9 @@ async def retry_txn(max_attempts: int = 20) -> AsyncIterator[AttemptManager]:
             stop=stop_after_attempt(max_attempts),
             retry=retry_if_exception_type(TryAgain) | retry_if_exception_type(DBAPIError),
         ):
-            # The caller of a Python generator cannot catch the exceptions thrown in the block,
-            # so we need to pass AttemptManager.
+            # Since Python generators cannot catch the exceptions thrown in the code block executed
+            # when yielded because stack frames are switched, we should pass AttemptManager to
+            # provide a shared exception handling mechanism like the original execute_with_retry().
             yield attempt
             assert attempt.retry_state.outcome is not None
             exc = attempt.retry_state.outcome.exception()

src/ai/backend/manager/scheduler/types.py

@@ -437,7 +437,7 @@ def prioritize(pending_sessions: Sequence[SessionRow]) -> tuple[int, list[Sessio
             return -1, []
         priorities = {s.priority for s in pending_sessions}
         assert len(priorities) > 0
-        top_priority = sorted(priorities, reverse=True)[0]
+        top_priority = max(priorities)
         return top_priority, [*filter(lambda s: s.priority == top_priority, pending_sessions)]
 
     @abstractmethod