fix: multi-source nodes can be skipped under max_turns

[majiayu000]

Why are these changes needed?

When a node has multiple incoming edges, the current "all" activation only enqueues the target after the final parent runs. In a graph like P -> U, P -> E, U -> E with max_turns=2, E never runs because it would be scheduled in a third turn. This change queues the target once any parent triggers, so E can run in the second turn alongside U and avoids being skipped under tight turn budgets.

Repro

import asyncio
from autogen_agentchat.agents import BaseChatAgent
from autogen_agentchat.base import Response
from autogen_agentchat.messages import TextMessage
from autogen_agentchat.teams import GraphFlow
from autogen_agentchat.teams._group_chat._graph._digraph_group_chat import DiGraph, DiGraphEdge, DiGraphNode
from autogen_core import CancellationToken

class EchoAgent(BaseChatAgent):
    def __init__(self, name: str) -> None:
        super().__init__(name, f"Echo agent {name}")

    @property
    def produced_message_types(self):
        return (TextMessage,)

    async def on_messages(self, messages, cancellation_token: CancellationToken) -> Response:
        return Response(chat_message=TextMessage(content="ping", source=self.name))

    async def on_reset(self, cancellation_token: CancellationToken) -> None:
        pass

async def main():
    p = EchoAgent("P")
    u = EchoAgent("U")
    e = EchoAgent("E")
    graph = DiGraph(
        nodes={
            "P": DiGraphNode(name="P", edges=[DiGraphEdge(target="U"), DiGraphEdge(target="E")]),
            "U": DiGraphNode(name="U", edges=[DiGraphEdge(target="E")]),
            "E": DiGraphNode(name="E", edges=[]),
        }
    )
    team = GraphFlow(participants=[p, u, e], graph=graph, max_turns=2)
    result = await team.run(task="Start")
    print([m.source for m in result.messages])

asyncio.run(main())

Output (before):

['user', 'P', 'U']  # E missing because max_turns=2 is reached

Output (after):

['user', 'P', 'U', 'E']  # U/E order may vary

Related issue number

• N/A

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.