* fix: filter empty assistant messages to prevent 400 error on strict APIs
Some OpenAI-compatible APIs (e.g., Moonshot) reject requests with
empty content in assistant messages when no tool_calls are present.
This fix cleans up the messages payload before sending to avoid
'message at position X must not be empty' errors.
Closes related issue with fallback provider behavior.
* test(openai): add tests for empty assistant message filtering
* refactor(openai): simplify empty assistant message filtering logic
* style: format code
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Co-authored-by: RC-CHN <1051989940@qq.com>