ai-proxy Bedrock: Claude Opus 5.5 的聊天响应在推理时会丢失 `content` 字段内容是第一个块

Thanks for the Bedrock Converse driver (#13354) and for the multi-part Gemini handling in #14785. The same first-block pattern hits Bedrock, intermittently. Claude Opus 5.5 (and Opus 5) on Converse often put a `reasoningContent` block before the `text` block without thinking being requested; how often depends on the prompt. `from_bedrock_chat_openai` reads only `content[1].text`, so those `LLM/v1/chat` responses have no `content`. A quick `Say hi` comes back text-only and works.