修复同一对话中重复发送相同用户提示词时，因历史去重逻辑误判而不再追加本轮 user 消息、导致请求以 assistant 结尾并触发 Claude 4.6+ 返回 400（assistant-prefill final message is not supported）的问题；现仅在尾部已是相同 user 内容时才跳过追加，重复提示词可正常续聊。
Fix repeated identical user prompts in the same conversation failing with Claude 4.6+ 400 (assistant-prefill final message is not supported) because dedup logic skipped appending the new user turn when matching text appeared earlier; append is now skipped only when the tail message is already that user content, so identical prompts can continue normally.