fix: improve context length error detection for PPIO platform compatibility

- Extend error detection to handle PPIO's error message format:
  'The input is longer than the model's context length'
- Add case-insensitive matching using .lower() for robustness
- Maintain backward compatibility with existing 'maximum context length' check

This fixes the issue where PPIO platform models (e.g., ppio/zai-org/glm-5-turbo)
would fail with AgentState.ERROR due to unrecognized context length errors.

Author: leonforcode

astrbot/core/provider/sources/openai_source.py

@@ -1069,7 +1069,7 @@ async def _handle_api_error(
                     image_fallback_used,
                 )
             raise e
-        if "maximum context length" in str(e):
+        if "maximum context length" in str(e) or "context length" in str(e).lower():
             logger.warning(
                 f"上下文长度超过限制。尝试弹出最早的记录然后重试。当前记录条数: {len(context_query)}",
             )