workflow self loop not work

[ZenWayne]

What happened?

Describe the bug
when use self loop, workflow not work

To Reproduce
add the following test case to test_group_chat_graph.py

@pytest.mark.asyncio
async def test_digraph_group_chat_loop_with_exit_condition_2(runtime: AgentRuntime | None) -> None:
    # Agents A and C: Echo Agents
    agent_a = _EchoAgent("A", description="Echo agent A")
    agent_c = _EchoAgent("C", description="Echo agent C")

    # Replay model client for agent B
    model_client = ReplayChatCompletionClient(
        chat_completions=[
            "loop",  # First time B will ask to loop
            "loop",  # Second time B will ask to loop
            "exit",  # Third time B will say exit
        ]
    )
    # Agent B: Assistant Agent using Replay Client
    agent_b = AssistantAgent("B", description="Decision agent B", model_client=model_client)

    # DiGraph: A → B(self looping) → C (conditional back to A or terminate)
    graph = DiGraph(
        nodes={
            "A": DiGraphNode(name="A", edges=[DiGraphEdge(target="B")]),
            "B": DiGraphNode(
                name="B", edges=[DiGraphEdge(target="C", condition="exit"), DiGraphEdge(target="B", condition="loop")]
            ),
            "C": DiGraphNode(name="C", edges=[]),
        },
        default_start_node="A",
    )

    team = GraphFlow(
        participants=[agent_a, agent_b, agent_c],
        graph=graph,
        runtime=runtime,
        termination_condition=MaxMessageTermination(20),
    )

    # Run
    result = await team.run(task="Start")

    # Assert message order
    expected_sources = [
        "user",
        "A",
        "B",  # 1st loop
        "B", # 2nd loop  
        "B",
        "C",
        _DIGRAPH_STOP_AGENT_NAME,
    ]

    actual_sources = [m.source for m in result.messages]

    assert actual_sources == expected_sources
    assert result.stop_reason is not None
    assert result.messages[-2].source == "C"
    assert any(m.content == "exit" for m in result.messages[:-1])  # type: ignore[attr-defined,union-attr]
    assert result.messages[-1].source == _DIGRAPH_STOP_AGENT_NAME

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Expected behavior
test case passed

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Additional context
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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

3.12

.NET version

None

Operating system

Windows