fix(handoffs): await on_handoff callables with async __call__

[adityasingh2400]

Summary

handoff(on_handoff=...) accepts any callable, but the dispatch in
src/agents/handoffs/__init__.py decided whether to await based on
inspect.iscoroutinefunction(on_handoff). That predicate returns False for
class instances whose __call__ is async def, so for callbacks such as

class MyHook:
    async def __call__(self, ctx, input):
        ...

handoff(agent, input_type=Foo, on_handoff=MyHook())

the runtime called MyHook()(ctx, input), got a coroutine back, and discarded
it without awaiting. Result: the user's on_handoff never executed and Python
emitted RuntimeWarning: coroutine '__call__' was never awaited.

The fix mirrors the pattern already used elsewhere in the codebase (e.g.
Handoff._is_enabled, tool.py): invoke the callable, then await the
result iff it's awaitable. Sync callables remain synchronous; native async
functions and async-__call__ instances are now awaited correctly.

Test plan

• New test test_callable_class_with_async_dunder_call_is_awaited covers
  both the input_type-bearing branch and the no-input branch.
• pytest tests/test_handoff_tool.py tests/test_handoff_history_duplication.py tests/test_handoff_prompt.py (28 passed).
• ruff check + ruff format --check clean on the touched files.
• Confirmed the new test fails on main with the documented RuntimeWarning before applying the fix.

[seratch]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

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