Stop consuming output queue when there is an exception from agent when using custom runtime

[ekzhu]

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Issue body

Make sure this test work for runtime fixture:

autogen/python/packages/autogen-agentchat/tests/test_group_chat.py

Lines 346 to 363 in 7e5c115

	# TODO: add runtime fixture for testing with custom runtime once the issue regarding
	# hanging on exception is resolved.
	@pytest.mark.asyncio
	async def test_round_robin_group_chat_with_exception_raised() -> None:
	agent_1 = _EchoAgent("agent_1", description="echo agent 1")
	agent_2 = _FlakyAgent("agent_2", description="echo agent 2")
	agent_3 = _EchoAgent("agent_3", description="echo agent 3")
	termination = MaxMessageTermination(3)
	team = RoundRobinGroupChat(
	participants=[agent_1, agent_2, agent_3],
	termination_condition=termination,
	)
	with pytest.raises(ValueError, match="I am a flaky agent..."):
	await team.run(
	task="Write a program that prints 'Hello, world!'",
	)