Ollama throw "Object of type Message is not JSON serializable" error

### What happened?

**Describe the bug**
When testing the code in https://microsoft.github.io/autogen/stable/user-guide/agentchat-user-guide/tutorial/models.html#ollama-experimental with logging enabled. 
It show error: **TypeError: Object of type Message is not JSON serializable**

**To Reproduce**
Ollama is installed.
```python
from autogen_core.models import UserMessage
from autogen_ext.models.ollama import OllamaChatCompletionClient
import asyncio
import logging

# Configure logging
logging.basicConfig(
    level=logging.INFO,
    format='%(asctime)s - %(name)s - %(levelname)s - %(message)s',
    datefmt='%Y-%m-%d %H:%M:%S'
)
logger = logging.getLogger(__name__)

async def main():
    ollama_model_client = OllamaChatCompletionClient(model="llama3.2")
    response = await ollama_model_client.create([UserMessage(content="What is the capital of France?", source="user")])
    logger.info(f"Sending message: {response}")

asyncio.run(main())
```

The output is as below:
```
2025-03-12 22:15:56 - autogen_core.events - INFO - Dropped the following unrecognized keys from create_args: []
2025-03-12 22:15:57 - httpx - INFO - HTTP Request: POST http://127.0.0.1:11434/api/chat "HTTP/1.1 200 OK"
--- Logging error ---
Traceback (most recent call last):
  File ".pyenv/versions/3.12.8/lib/python3.12/logging/__init__.py", line 1160, in emit
    msg = self.format(record)
          ^^^^^^^^^^^^^^^^^^^
  File ".pyenv/versions/3.12.8/lib/python3.12/logging/__init__.py", line 999, in format
    return fmt.format(record)
           ^^^^^^^^^^^^^^^^^^
  File ".pyenv/versions/3.12.8/lib/python3.12/logging/__init__.py", line 703, in format
    record.message = record.getMessage()
                     ^^^^^^^^^^^^^^^^^^^
  File ".pyenv/versions/3.12.8/lib/python3.12/logging/__init__.py", line 390, in getMessage
    msg = str(self.msg)
          ^^^^^^^^^^^^^
  File "venv/lib/python3.12/site-packages/autogen_core/logging.py", line 64, in __str__
    return json.dumps(self.kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File ".pyenv/versions/3.12.8/lib/python3.12/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".pyenv/versions/3.12.8/lib/python3.12/json/encoder.py", line 200, in encode
    chunks = self.iterencode(o, _one_shot=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".pyenv/versions/3.12.8/lib/python3.12/json/encoder.py", line 258, in iterencode
    return _iterencode(o, 0)
           ^^^^^^^^^^^^^^^^^
  File ".pyenv/versions/3.12.8/lib/python3.12/json/encoder.py", line 180, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type Message is not JSON serializable
Call stack:
  File "tests/test_ollama.py", line 21, in <module>
    asyncio.run(main())
  File ".pyenv/versions/3.12.8/lib/python3.12/asyncio/runners.py", line 194, in run
    return runner.run(main)
  File ".pyenv/versions/3.12.8/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
  File ".pyenv/versions/3.12.8/lib/python3.12/asyncio/base_events.py", line 673, in run_until_complete
    self.run_forever()
  File ".pyenv/versions/3.12.8/lib/python3.12/asyncio/base_events.py", line 640, in run_forever
    self._run_once()
  File ".pyenv/versions/3.12.8/lib/python3.12/asyncio/base_events.py", line 1992, in _run_once
    handle._run()
  File ".pyenv/versions/3.12.8/lib/python3.12/asyncio/events.py", line 88, in _run
    self._context.run(self._callback, *self._args)
  File "tests/test_ollama.py", line 17, in main
    response = await ollama_model_client.create([UserMessage(content="What is the capital of France?", source="user")])
  File "venv/lib/python3.12/site-packages/autogen_ext/models/ollama/_ollama_client.py", line 485, in create
    logger.info(
Message: <autogen_core.logging.LLMCallEvent object at 0x10711f650>
Arguments: ()
2025-03-12 22:15:57 - __main__ - INFO - Sending message: finish_reason='unknown' content='The capital of France is Paris.' usage=RequestUsage(prompt_tokens=32, completion_tokens=8) cached=False logprobs=None thought=None
```

**Expected behavior**
Do not show any error in log.

### Which packages was the bug in?

Python Core (autogen-core)

### AutoGen library version.

Python dev (main branch)

### Other library version.

_No response_

### Model used

llama3.2

### Model provider

Ollama

### Other model provider

_No response_

### Python version

3.12

### .NET version

None

### Operating system

MacOS

Labels: needs-triage
State: closed