[Bugfix] Fix multi-round chat error when mistral tokenizer is used (#12859)

Signed-off-by: Zifei Tong <zifeitong@gmail.com>
Co-authored-by: Cyrus Leung <cyrus.tl.leung@gmail.com>

vllm/transformers_utils/tokenizers/mistral.py

@@ -291,6 +291,16 @@ class MistralTokenizer:
 
         from mistral_common.protocol.instruct.request import (
             ChatCompletionRequest)
+
+        # mistral-common requires AssistantMessage content to be string [1].
+        #
+        # [1]: https://github.com/mistralai/mistral-common/blob/f4a06998b75ed78bbf5aaf569590b772ea26c9f6/src/mistral_common/protocol/instruct/messages.py#L80
+        for message in messages:
+            if message.get("role") == "assistant":
+                content = message.get("content")
+                if isinstance(content, list):
+                    content = "\n".join(chunk.get("text") for chunk in content)
+                    message["content"] = content
         request = ChatCompletionRequest(messages=messages,
                                         tools=tools)  # type: ignore[type-var]
         encoded = self.mistral.encode_chat_completion(request)