Realtime Handoff is_enabled doesn't work with callable

[mcbortolozzo]

Describe the bug

Passing a callable to the is_enabled to a Handoff of a Realtime Agent doesn't work because RealtimeAgent is not properly imported to the realtime handoffs file.

    async def _is_enabled(ctx: RunContextWrapper[Any], agent_base: AgentBase[Any]) -> bool:
        assert callable(is_enabled), "is_enabled must be non-null here"
>       assert isinstance(agent_base, RealtimeAgent), "Can't handoff to a non-RealtimeAgent"
                                      ^^^^^^^^^^^^^
E       NameError: name 'RealtimeAgent' is not defined

src/agents/realtime/handoffs.py:151: NameError

Debug information

• Agents SDK version: main branch
• Python 3.11

Repro steps

Remove the following lines from this test, which are forcing it to pass. This is the same issue that occurs when attempting to run the realtime handoff with a callable in a real application.

openai-agents-python/tests/realtime/test_realtime_handoffs.py

Lines 142 to 148 in 095496e

	# Patch missing symbol in module to satisfy isinstance in closure
	import agents.realtime.handoffs as rh
	if not hasattr(rh, "RealtimeAgent"):
	from agents.realtime import RealtimeAgent as _RT
	rh.RealtimeAgent = _RT # type: ignore[attr-defined]

Expected behavior

The Handoff should work as expected with a callable.