Cycle detected without exit condition: generator -> reviewer -> generator

### What happened?

```python
import sys
import os
sys.path.append(os.path.dirname(os.path.dirname(__file__)))
from autogen_agentchat.agents import AssistantAgent, MessageFilterAgent, MessageFilterConfig, PerSourceFilter
from autogen_agentchat.teams import DiGraphBuilder, GraphFlow
from autogen_agentchat.ui import Console
from autogen_ext.models.openai import OpenAIChatCompletionClient
import asyncio  
from app.agentmanager import model_client_deep, model_client_fast
from autogen_agentchat.conditions import ExternalTermination, TextMentionTermination, MaxMessageTermination

from autogen_agentchat.agents import AssistantAgent, MessageFilterAgent, MessageFilterConfig, PerSourceFilter
from autogen_agentchat.teams import (
    DiGraphBuilder,
    GraphFlow,
)
from autogen_agentchat.ui import Console
from autogen_ext.models.openai import OpenAIChatCompletionClient



async def main():
# Agents
    generator = AssistantAgent("generator", model_client=model_client_fast, system_message="Generate a list of creative ideas.")
    reviewer = AssistantAgent(
        "reviewer",
        model_client=model_client_fast,
        system_message="Review ideas and provide feedbacks, or just 'APPROVE' for final approval.",
    )
    summarizer_core = AssistantAgent(
        "summary", model_client=model_client_fast, system_message="Summarize the user request and the final feedback."
    )

    # Filtered summarizer
    filtered_summarizer = MessageFilterAgent(
        name="summary",
        wrapped_agent=summarizer_core,
        filter=MessageFilterConfig(
            per_source=[
                PerSourceFilter(source="user", position="first", count=1),
                PerSourceFilter(source="reviewer", position="last", count=1),
            ]
        ),
    )



    # Build graph with conditional loop
    builder = DiGraphBuilder()
    builder.add_node(generator).add_node(reviewer).add_node(filtered_summarizer)
    builder.add_edge(generator, reviewer)
    builder.add_edge(reviewer, filtered_summarizer, condition=lambda msg: "APPROVE" in msg.to_model_text())
    builder.add_edge(reviewer, generator, condition=lambda msg: "APPROVE" not in msg.to_model_text())
    builder.set_entry_point(generator)  # Set entry point to generator. Required if there are no source nodes.
    graph = builder.build()

    # Create the flow
    flow = GraphFlow(
        participants=builder.get_participants(),
        graph=graph,
    )


    async for event in flow.run_stream(task="Brainstorm ways to reduce plastic waste."):
        print(event)


asyncio.run(main())

```

**Describe the bug**
I already update my autogen to version 0.6.0,Python 3.12.10 ,and I use the code which in offical document demo.
ValueError: Cycle detected without exit condition: generator -> reviewer -> generator

**To Reproduce**
.- just run the code.the error will appear

**Expected behavior**
find the root cause,why  the code in my computer can not get the same result as demo ?

**Screenshots**
(.venv) vscode ➜ /myautogenprj (master) $ /myautogenprj/.venv/bin/python /myautogenprj/myapp/test2.py
Traceback (most recent call last):
  File "/myautogenprj/myapp/test2.py", line 69, in <module>
    asyncio.run(main())
  File "/usr/lib/python3.12/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/base_events.py", line 691, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/myautogenprj/myapp/test2.py", line 56, in main
    graph = builder.build()
            ^^^^^^^^^^^^^^^
  File "/myautogenprj/.venv/lib/python3.12/site-packages/autogen_agentchat/teams/_group_chat/_graph/_graph_builder.py", line 175, in build
    graph.graph_validate()
  File "/myautogenprj/.venv/lib/python3.12/site-packages/autogen_agentchat/teams/_group_chat/_graph/_digraph_group_chat.py", line 206, in graph_validate
    self._has_cycles = self.has_cycles_with_exit()
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/myautogenprj/.venv/lib/python3.12/site-packages/autogen_agentchat/teams/_group_chat/_graph/_digraph_group_chat.py", line 175, in has_cycles_with_exit
    if dfs(node):
       ^^^^^^^^^
  File "/myautogenprj/.venv/lib/python3.12/site-packages/autogen_agentchat/teams/_group_chat/_graph/_digraph_group_chat.py", line 153, in dfs
    if dfs(target):
       ^^^^^^^^^^^
  File "/myautogenprj/.venv/lib/python3.12/site-packages/autogen_agentchat/teams/_group_chat/_graph/_digraph_group_chat.py", line 163, in dfs
    raise ValueError(
ValueError: Cycle detected without exit condition: generator -> reviewer -> generator

**Additional context**
Add any other context about the problem here.


### Which packages was the bug in?

Python AgentChat (autogen-agentchat>=0.4.0)

### AutoGen library version.

Python 0.6.0

### Other library version.

_No response_

### Model used

gemini 2.5 pro

### Model provider

None

### Other model provider

_No response_

### Python version

3.12

### .NET version

None

### Operating system

Windows

Labels: needs-triage
State: closed