Could not send Multiple system message at autogen-ext/.../anthropic/_anthropic_client.py

### What happened?

**Describe the bug**
Anthropic sdk aware code(autogen-ext/../model/anthropic) could not work with multiple system message.
However it could take it.

**To Reproduce**
* Clinets *
```python
clients =  [
    anthropic_client,
    openai_client,
    anthropic_openai_client,
    gemini_openai_client,
]
```

```python
messages = [
    SystemMessage(content="Must say foo too"),
    SystemMessage(content="Must say bar too"),
    UserMessage(content="tell", source="user"),
]

for client in clients:
    try:
        text = asyncio.run(client.create(messages=messages))
        print(text.content, client.model_info.get("family"))
    except Exception as e:
        print("ERROR", e, end=" ")
        print(f"Error with {client.model_info.get('family')}")
```

the output is that.
```
ERROR Multiple system messages are not supported Error with claude-3.7-sonnet
I'm here to help! What would you like to know or talk about today? Foo and bar, perhaps? gpt-4o
I need to say both "foo" and "bar" because those are the instructions from the prompt. So:

foo
bar claude-3.7-sonnet
Bar too!
 gemini-2.0-flash
```

we could know that, JUST ANTHROPIC is could not work with`multiple system message`.
So, I was change to _anthropic_client.py like it.

```python
        for message in messages:
            if isinstance(message, SystemMessage):
                if system_message is not None:
                    pass
                    # raise ValueError("Multiple system messages are not supported")
                system_message = to_anthropic_type(message)
            else:
                anthropic_message = to_anthropic_type(message)
                if isinstance(anthropic_message, list):
                    anthropic_messages.extend(anthropic_message)
                elif isinstance(anthropic_message, str):
                    msg = MessageParam(
                        role="user" if isinstance(message, UserMessage) else "assistant", content=anthropic_message
                    )
                    anthropic_messages.append(msg)
                else:
                    anthropic_messages.append(anthropic_message)

```

??? WOW IT WORKS COLLECT!
(I do not know about It works, why?, cause, Anthropic SDK case system_message is change at last system message, just delete that raise)
```
I need to say both "foo" and "bar" in my response, so here they are: foo bar.

Is there something specific you'd like me to tell you about? I'm happy to help with information, explanations, or other assistance. claude-3.7-sonnet
foo bar gpt-4o
I understand you're asking me to tell you something, but I need a bit more guidance on what specifically you'd like me to tell you about. Could you please provide more details about what information you're looking for? I'd be happy to help once I understand your request better. claude-3.7-sonnet
Bar too.
 gemini-2.0-flash
```

and they say "foo" and "bar" too. So we could know it.
-> multiple system message is working now.

**Expected behavior**
Merge contents each other when multiple system message

Could I know about any information of why adding it?
-> `SocietyOfMindAgent` was makes two system message, so now Anthropic could not using that Agent.


### Which packages was the bug in?

Python Extensions (autogen-ext)

### AutoGen library version.

Python dev (main branch)

### Other library version.

_No response_

### Model used

_No response_

### Model provider

None

### Other model provider

_No response_

### Python version

None

### .NET version

None

### Operating system

None

Labels: needs-triage
State: closed