[BUG] AiMessage text content is not copied when toolCalls are present

[langchain4j]

Describe the bug
When AiMessage contains both text and toolCalls, next time it is sent to the LLM again in a list of messages, only toolCalls is present.

Log and Stack trace
Response:

{
  "id": "chatcmpl-9Fel2FjEBshnmq0c4wiEtOgnJRHnT",
  "object": "chat.completion",
  "created": 1713518620,
  "model": "gpt-3.5-turbo-0125",
  "choices": [
    {
      "index": 0,
      "message": {
        "role": "assistant",
        "content": "I encountered an issue while trying to add the word \"haben\" to your vocabulary. Let me try that again.",
        "tool_calls": [
          {
            "id": "call_nZ6wMhj9MXkch4zBPKGcd3xS",
            "type": "function",
            "function": {
              "name": "addWordToVocabulary",
              "arguments": "{\"arg1\":\"haben\"}"
            }
          }
        ]
      },
      "logprobs": null,
      "finish_reason": "tool_calls"
    }
  ],
  "usage": {
    "prompt_tokens": 109,
    "completion_tokens": 44,
    "total_tokens": 153
  },
  "system_fingerprint": "fp_d9767fc5b9"
}

Next request:

{
  "model": "gpt-3.5-turbo",
  "messages": [
    {
      "role": "system",
      "content": "You are a tutor of a German language."
    },
    {
      "role": "user",
      "content": "add a word \"haben\" in my vocab"
    },
    {
      "role": "assistant",
      "content": null,
      "tool_calls": [
        {
          "id": "call_h6DsefXXfv2kgacwW1M9MkBw",
          "type": "function",
          "function": {
            "name": "addWordToVocabulary",
            "arguments": "{\"arg1\":\"haben\"}"
          }
        }
      ]
    },
    {
      "role": "tool",
      "tool_call_id": "call_h6DsefXXfv2kgacwW1M9MkBw",
      "content": "DB was closed"
    },
    {
      "role": "assistant",
      "content": null,
      "tool_calls": [
        {
          "id": "call_nZ6wMhj9MXkch4zBPKGcd3xS",
          "type": "function",
          "function": {
            "name": "addWordToVocabulary",
            "arguments": "{\"arg1\":\"haben\"}"
          }
        }
      ]
    },
    {
      "role": "tool",
      "tool_call_id": "call_nZ6wMhj9MXkch4zBPKGcd3xS",
      "content": "DB was closed"
    }
  ],
  "temperature": 0.7,
  "tools": [
    {
      "type": "function",
      "function": {
        "name": "addWordToVocabulary",
        "description": "",
        "parameters": {
          "type": "object",
          "properties": {
            "arg1": {
              "type": "string"
            }
          },
          "required": [
            "arg1"
          ]
        }
      }
    },
    {
      "type": "function",
      "function": {
        "name": "startLesson",
        "description": "",
        "parameters": {
          "type": "object",
          "properties": {},
          "required": []
        }
      }
    },
    {
      "type": "function",
      "function": {
        "name": "finishLesson",
        "description": "",
        "parameters": {
          "type": "object",
          "properties": {},
          "required": []
        }
      }
    }
  ]
}

To Reproduce
Return an error from a tool call, so that LLM needs to retry.

Expected behavior
Text content should not be lost.

Please complete the following information:

• LangChain4j version: 0.30.0
• LLM(s) used: openai
• Java version: 21