Issue function calling in AssistantAgent with Gemini model

### What happened?

The issue is that AssistantAgent with Gemini model does not return `call_id`s after function calling. In case there are multiple function callings,  it fails to correlate tool call results with tool calls.

I am using the latest version (v0.4.6) and `OpenAIChatCompletionClient` from `autogen_ext.models.openai`

It is block [here](https://github.com/microsoft/autogen/blob/392aa144913b5f17f85a61329901715240b4f44c/python/packages/autogen-agentchat/src/autogen_agentchat/agents/_assistant_agent.py#L454C9-L466C1)
```python

        tool_calls = [call for call in model_result.content if call.name not in self._handoffs]
        tool_call_results: List[FunctionExecutionResult] = []
        for tool_call in tool_calls:
            found = False
            for exec_result in exec_results:
                if exec_result.call_id == tool_call.id:
                    found = True
                    tool_call_results.append(exec_result)
                    break
            if not found:
                raise RuntimeError(f"Tool call result not found for call id: {tool_call.id}")
```

### What did you expect to happen?

I am not sure if this is an issue of autogen itself or issue from Google Genimi.

tool calls and results are correlated correct. If there is not id, then should be processed different.  

### How can we reproduce it (as minimally and precisely as possible)?

# define two functions

```python
async def get_current_time() -> str:
    return "The current time is 12:00 PM."

async def get_stock_price(stock: str ="MFST") -> str:
    return "The stock price is 500 dollors."

tools: List[Tool] = [
    FunctionTool(
        get_current_time,
        name="get_current_time",
        description="Get current time in Paris.",
    ),
    FunctionTool(
        get_stock_price,
        name="get_stock_price",
        description="Fetch the stock price for a given company."
    ),
]
mode_client = OpenAIChatCompletionClient(
        model="gemini-2.0-flash-exp",
        base_url="https://generativelanguage.googleapis.com/v1beta/openai/",
        api_key="",
        model_info={
            "vision": True,
            "function_calling": True,
            "json_output": True,
            "family": "unknown",
        },
    )
response = await AssistantAgent(
        name="Agent",
        model_client=model_client,
        tools=tools,
        system_message= "You are an assistant with some tools that can be used to answer some questions",
    ).on_messages([TextMessage(content="What is current time of Paris now? And what is stock price of MSFT", source="user")], CancellationToken())
print(response)
```

results:
````
Response(chat_message=ToolCallSummaryMessage(source='Agent', models_usage=None, content='The current time is 12:00 PM.\nThe current time is 12:00 PM.', type='ToolCallSummaryMessage'), inner_messages=[ToolCallRequestEvent(source='Agent', models_usage=RequestUsage(prompt_tokens=58, completion_tokens=13), content=[FunctionCall(id='', arguments='{}', name='get_current_time'), FunctionCall(id='', arguments='{"stock":"MSFT"}', name='get_stock_price')], type='ToolCallRequestEvent'), ToolCallExecutionEvent(source='Agent', models_usage=None, content=[FunctionExecutionResult(content='The current time is 12:00 PM.', call_id='', is_error=False), FunctionExecutionResult(content='The stock price is 500 dollors.', call_id='', is_error=False)], type='ToolCallExecutionEvent')])
````

### AutoGen version

0.4.6

### Which package was this bug in

AgentChat

### Model used

gemini

### Python version

3.10

### Operating system

ubuntu

### Any additional info you think would be helpful for fixing this bug

_No response_

Labels: proj-agentchat
State: closed