Covers the #16748 fix:
- unsigned thinking blocks synthesised from reasoning_content survive replay
- non-latest assistant turns keep their thinking (DeepSeek validates every turn)
- signed Anthropic blocks are stripped (DeepSeek can't validate them)
- cache_control is stripped from thinking blocks
- OpenAI-compat base (api.deepseek.com without /anthropic) is NOT matched
- non-DeepSeek third parties (minimax) keep the generic strip-all behaviour