fix: prevent ValueError when removing already-removed API key in retry loop (#6193)

In _handle_api_error(), when a 429 rate-limit is encountered, the code
calls available_api_keys.remove(chosen_key). If the same key was already
removed in a previous retry iteration (e.g. the key rotated back to the
same value), this raises ValueError which crashes the entire LLM request
with an opaque error instead of a proper retry/fallback.

Add a membership check before calling remove() to prevent the crash.

Co-authored-by: easonysliu <easonysliu@tencent.com>

astrbot/core/provider/sources/openai_source.py

@@ -629,7 +629,8 @@ class ProviderOpenAIOfficial(Provider):
             # 最后一次不等待
             if retry_cnt < max_retries - 1:
                 await asyncio.sleep(1)
-            available_api_keys.remove(chosen_key)
+            if chosen_key in available_api_keys:
+                available_api_keys.remove(chosen_key)
             if len(available_api_keys) > 0:
                 chosen_key = random.choice(available_api_keys)
                 return (