Bug: GraphFlow cannot be serialized or deserialized

### What happened?

**Describe the bug**
When trying to serialize (dump_component()) a GraphFlow instance and then deserialize (load_component()), an error occurs:
```
NotImplementedError: This component does not support dumping to config
```
This suggests that some internal components used in GraphFlow do not support serialization yet.

**To Reproduce**
Steps to reproduce the behavior:
```python
import asyncio
from autogen_agentchat.ui import Console
from autogen_agentchat.agents import AssistantAgent
from autogen_agentchat.teams import DiGraphBuilder, GraphFlow
from autogen_ext.models.openai import OpenAIChatCompletionClient

async def main():
    client = OpenAIChatCompletionClient(model="gpt-4.1-nano")

    writer = AssistantAgent("writer", model_client=client, system_message="Draft a short paragraph on climate change.")
    reviewer = AssistantAgent("reviewer", model_client=client, system_message="Review the draft and suggest improvements.")

    builder = DiGraphBuilder()
    builder.add_node(writer).add_node(reviewer)
    builder.add_edge(writer, reviewer)

    graph = builder.build()

    flow = GraphFlow([writer, reviewer], graph=graph)
    await Console(flow.run_stream(task="Draft a short paragraph on climate change."))

    serialized_flow = flow.dump_component()  # 🔥 Fails here
    print(serialized_flow.json())

    deserialized_flow = GraphFlow.load_component(serialized_flow)
    await Console(deserialized_flow.run_stream(task="Draft a short paragraph on climate change."))

asyncio.run(main())
```
Stacktrace
```
File "errortest.py", line 1404, in main
    serialized_flow = flow.dump_component()
                      ^^^^^^^^^^^^^^^^^^^^^
File "autogen_core/_component_config.py", line 150, in dump_component
    obj_config = self._to_config().model_dump(exclude_none=True)
                 ^^^^^^^^^^^^^^^^^
File "autogen_agentchat/teams/_group_chat/_graph/_digraph_group_chat.py", line 703, in _to_config
    participants = [participant.dump_component() for participant in self._participants]
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "autogen_agentchat/teams/_group_chat/_graph/_digraph_group_chat.py", line 703, in <listcomp>
    participants = [participant.dump_component() for participant in self._participants]
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "autogen_core/_component_config.py", line 150, in dump_component
    obj_config = self._to_config().model_dump(exclude_none=True)
                 ^^^^^^^^^^^^^^^^^
File "autogen_core/_component_config.py", line 114, in _to_config
    raise NotImplementedError("This component does not support dumping to config")
NotImplementedError: This component does not support dumping to config
```

**Expected behavior**
- GraphFlow should be fully serializable and deserializable via dump_component() and load_component().
- All participants and internal graph structures should be dumped and reconstructed properly.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Additional context**
- According to the error trace, one of the participants (AssistantAgent) or related components likely lacks _to_config() implementation.
- It may also be related to how DiGraph itself is serialized inside GraphFlow.
- Other group chat types have improved serialization support, but GraphFlow seems missing such coverage.
- This issue blocks using GraphFlow for distributed session management and saving/restoring team states.


### Which packages was the bug in?

Python AgentChat (autogen-agentchat>=0.4.0)

### 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