fix: close unawaited reset coroutine on early return (#5033)

When an OnLLMRequestEvent hook stops event propagation, the
reset_coro created by build_main_agent was never awaited, causing
a RuntimeWarning. Close the coroutine explicitly before returning.

Fixes #5032

Co-authored-by: Limitless2023 <limitless@users.noreply.github.com>

astrbot/core/pipeline/process_stage/method/agent_sub_stages/internal.py

@@ -190,6 +190,8 @@ class InternalAgentSubStage(Stage):
                 )
 
                 if await call_event_hook(event, EventType.OnLLMRequestEvent, req):
+                    if reset_coro:
+                        reset_coro.close()
                     return
 
                 # apply reset