Issue: TypeError in MultimodalWebSurfer (and maybe.. sending Image too) when using Anthropic models (at newist version of anthropic)

### What happened?

**Describe the bug**
When using the MultimodalWebSurfer with an Anthropic model, a TypeError: Cannot instantiate typing.Union occurs. This error arises because the typing.Union type hint is being incorrectly used as a class constructor within the _anthropic_client.py file, specifically when attempting to convert a user message to the format required by the Anthropic API(at newist version:=after 0.48.0 of anthropic).

**To Reproduce**
```python
from autogen_ext.agents.web_surfer import MultimodalWebSurfer
from autogen_ext.models.anthropic import AnthropicChatCompletionClient
from autogen_agentchat.messages import TextMessage, MultiModalMessage
from autogen_agentchat.ui import Console
from autogen_core import CancellationToken
# Assuming you have an Anthropic API key set as an environment variable.

# Create an Anthropic model client.  The specific configuration
# (model name, etc.) might need adjustment to match your setup.

anthropic_client = AnthropicChatCompletionClient(
    model="claude-3-7-sonnet-latest",
    api_key="[OWN API KEY]",
)

# Create a MultimodalWebSurfer_agent.
agent = MultimodalWebSurfer(name="MultimodalWebSurfer", model_client=anthropic_client)

# Attempt to send a message that requires multimodal processing (e.g., an image).
# The exact message structure will depend on how the agent is used, but the
# key is that it needs to trigger the code path that leads to the TypeError.
# The following is a placeholder; you'll likely need to adapt it.
async def assistant_run() -> None:
    response = await agent.on_messages(
        [
            TextMessage(content="Search 2025/03/01 weather", source="user"),
        ],
        cancellation_token=CancellationToken(),
    )
    print(response.inner_messages)
    print(response.chat_message)

import asyncio
asyncio.run(assistant_run())
```
*Stack Trace*
```
source='MultimodalWebSurfer' models_usage=None metadata={} content='Web surfing error:\n\nTraceback (most recent call last):\n  File "/home/cysong/work/TIPS/test-agent/.venv/lib/python3.12/site-packages/autogen_ext/agents/web_surfer/_multimodal_web_surfer.py", line 442, in on_messages_stream\n    content = await self._generate_reply(cancellation_token=cancellation_token)\n              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File "/home/cysong/work/TIPS/test-agent/.venv/lib/python3.12/site-packages/autogen_ext/agents/web_surfer/_multimodal_web_surfer.py", line 612, in _generate_reply\n    response = await self._model_client.create(\n               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File "/home/cysong/work/TIPS/test-agent/.venv/lib/python3.12/site-packages/autogen_ext/models/anthropic/_anthropic_client.py", line 451, in create\n    anthropic_message = to_anthropic_type(message)\n                        ^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File "/home/cysong/work/TIPS/test-agent/.venv/lib/python3.12/site-packages/autogen_ext/models/anthropic/_anthropic_client.py", line 256, in to_anthropic_type\n    return user_message_to_anthropic(message)\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File "/home/cysong/work/TIPS/test-agent/.venv/lib/python3.12/site-packages/autogen_ext/models/anthropic/_anthropic_client.py", line 165, in user_message_to_anthropic\n    source=Source(\n           ^^^^^^^\n  File "/home/cysong/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/typing.py", line 1184, in __call__\n    result = self.__origin__(*args, **kwargs)\n             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File "/home/cysong/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/typing.py", line 501, in __call__\n    raise TypeError(f"Cannot instantiate {self!r}")\nTypeError: Cannot instantiate typing.Union\n' type='TextMessage'
```

**Expected behavior**
The `MultimodalWebSurfer` should process multimodal inputs (including images) correctly when using an Anthropic model, without raising a TypeError.  The agent should be able to communicate with the Anthropic API and handle the response.

**Screenshots**
(Not applicable, as the error is a traceback.)

**Additional context**
Python version: 3.12 (from the traceback)
OS: Linux (from the traceback, x86_64) (It could reproduce at macOS too)
autogen_core.__version__: '0.4.9.2' 
autogen_ext.__version__: '0.4.9.2'
anthropic.__version__:'0.49.0'


### Which packages was the bug in?

Python Extensions (autogen-ext)

### AutoGen library version.

Other (please specify)

### Other library version.

Python 0.4.9.2

### Model used

claude-3-7-sonnet-latest

### Model provider

Anthropic

### Other model provider

_No response_

### Python version

3.12

### .NET version

None

### Operating system

Ubuntu

State: closed