[Bug]: ValidationError for RunCompletionEvent

### Describe the bug

When "reflection_with_llm" is used for the chat's summary and openai is not used, I get a "ValidationError":
`Input should be a valid string`. 


### Steps to reproduce

```python
from autogen import LLMConfig, ConversableAgent
model_name = "claude-3-sonnet-20240229"
llm_config = LLMConfig(api_type="anthropic", model=model_name)
agent1 = ConversableAgent(
    name="Agent1",
    llm_config=False,
)
agent2 = ConversableAgent(
    name="Agent2",
    system_message="You are a helpful assistant.",
    llm_config=llm_config,
)
response = agent1.run(
    agent2,
    max_turns=1,
    message="What is the capital of France?",
    summary_method="reflection_with_llm",
)
response.process()
```

### Model Used

claude-3-sonnet-20240229

### Expected Behavior

Not to raise.

### Screenshots and logs

Running the example above, gives:
```text
Agent1 (to Agent2):

What is the capital of France?

--------------------------------------------------------------------------------

>>>>>>>> USING AUTO REPLY...
Agent2 (to Agent1):

The capital of France is Paris.

--------------------------------------------------------------------------------

>>>>>>>> TERMINATING RUN (d22bbe12-270c-40fe-802d-356cc2ee6d1c): Maximum turns (1) reached
---------------------------------------------------------------------------
ValidationError                           Traceback (most recent call last)
Cell In[4], line 19
      8 agent2 = ConversableAgent(
      9     name="Agent2",
     10     system_message="You are a helpful assistant.",
     11     llm_config=llm_config,
     12 )
     13 response = agent1.run(
     14     agent2,
     15     max_turns=1,
     16     message="What is the capital of France?",
     17     summary_method="reflection_with_llm",
     18 )
---> 19 response.process()

File [~/.local/lib/python3.12/site-packages/autogen/io/run_response.py:199](https://dev.waldiez.io/home/waldiez/.local/lib/python3.12/site-packages/autogen/io/run_response.py#line=198), in RunResponse.process(self, processor)
    197 def process(self, processor: Optional[EventProcessorProtocol] = None) -> None:
    198     processor = processor or ConsoleEventProcessor()
--> 199     processor.process(self)

File [~/.local/lib/python3.12/site-packages/autogen/io/processors/console_event_processor.py:19](https://dev.waldiez.io/home/waldiez/.local/lib/python3.12/site-packages/autogen/io/processors/console_event_processor.py#line=18), in ConsoleEventProcessor.process(self, response)
     18 def process(self, response: "RunResponseProtocol") -> None:
---> 19     for event in response.events:
     20         self.process_event(event)

File [~/.local/lib/python3.12/site-packages/autogen/io/run_response.py:154](https://dev.waldiez.io/home/waldiez/.local/lib/python3.12/site-packages/autogen/io/run_response.py#line=153), in RunResponse._queue_generator(self, q)
    151         break
    153     if isinstance(event, ErrorEvent):
--> 154         raise event.content.error  # type: ignore[attr-defined]
    155 except queue.Empty:
    156     continue

File [~/.local/lib/python3.12/site-packages/autogen/agentchat/conversable_agent.py:1599](https://dev.waldiez.io/home/waldiez/.local/lib/python3.12/site-packages/autogen/agentchat/conversable_agent.py#line=1598), in ConversableAgent.run.<locals>.initiate_chat(self, iostream, response)
   1595     if hasattr(recipient, "last_speaker"):
   1596         _last_speaker = recipient.last_speaker
   1598     IOStream.get_default().send(
-> 1599         RunCompletionEvent(
   1600             history=chat_result.chat_history,
   1601             summary=chat_result.summary,
   1602             cost=chat_result.cost,
   1603             last_speaker=_last_speaker.name,
   1604         )
   1605     )
   1606 except Exception as e:
   1607     response.iostream.send(ErrorEvent(error=e))

File [~/.local/lib/python3.12/site-packages/autogen/events/base_event.py:71](https://dev.waldiez.io/home/waldiez/.local/lib/python3.12/site-packages/autogen/events/base_event.py#line=70), in wrap_event.<locals>.WrapperBase.__init__(self, *args, **data)
     69     super().__init__(*args, content=event_cls(*args, **data, content=content), **data)
     70 else:
---> 71     super().__init__(content=event_cls(*args, **data), **data)

File [~/.local/lib/python3.12/site-packages/autogen/events/base_event.py:23](https://dev.waldiez.io/home/waldiez/.local/lib/python3.12/site-packages/autogen/events/base_event.py#line=22), in BaseEvent.__init__(self, uuid, **kwargs)
     21 def __init__(self, uuid: Optional[UUID] = None, **kwargs: Any) -> None:
     22     uuid = uuid or uuid4()
---> 23     super().__init__(uuid=uuid, **kwargs)

File [~/.local/lib/python3.12/site-packages/pydantic/main.py:253](https://dev.waldiez.io/home/waldiez/.local/lib/python3.12/site-packages/pydantic/main.py#line=252), in BaseModel.__init__(self, **data)
    251 # `__tracebackhide__` tells pytest and some other tools to omit this function from tracebacks
    252 __tracebackhide__ = True
--> 253 validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
    254 if self is not validated_self:
    255     warnings.warn(
    256         'A custom validator is returning a value other than `self`.\n'
    257         "Returning anything other than `self` from a top level model validator isn't supported when validating via `__init__`.\n"
    258         'See the `model_validator` docs (https://docs.pydantic.dev/latest/concepts/validators/#model-validators) for more details.',
    259         stacklevel=2,
    260     )

ValidationError: 1 validation error for RunCompletionEvent
summary
  Input should be a valid string [type=string_type, input_value={'content': "Paris is a g...one, 'tool_calls': None}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/string_type
```

### Additional Information

- ag2 version: 0.9.6
- os: any
- python version: 3.13.3


Labels: bug
State: closed