fix: pre-bind handle variables to avoid Python 3.14 NameError in closure cells

[rmotgi1227]

Summary

Closes #1517.

Python 3.14 changed asyncio.Task to eagerly inspect a coroutine's closure cells when task.set_name() is called (CPython gh-126004). If a cell is unbound at that moment the interpreter raises:

NameError: cannot access free variable 'handle' where it is not
associated with a value in enclosing scope

Three methods in _workflow_instance.py had this pattern:

handle: _ActivityHandle          # annotation only — cell is EMPTY

async def run_activity() -> Any:
    handle._started = True       # closure over the empty cell

handle = _ActivityHandle(self, input, run_activity())
# ↑ constructor calls _register_task → task.set_name()
#   Python 3.14 inspects run_activity's cells → NameError

The same pattern exists in _outbound_start_child_workflow (_ChildWorkflowHandle) and _outbound_start_nexus_operation (_NexusOperationHandle).

Fix

Replace the bare annotation with a cast-annotated assignment:

handle = cast(_ActivityHandle, None)   # cell is now BOUND (to None)

cast is already imported; it is a no-op at runtime (returns its second argument) but preserves the static type. By the time the coroutine actually runs, the outer handle = _ActivityHandle(...) line has completed and the variable holds the real object, so no code ever observes the temporary None.

Changes

temporalio/worker/_workflow_instance.py

Location	Handle type	Change
_outbound_schedule_activity	_ActivityHandle	handle: _ActivityHandle → handle = cast(_ActivityHandle, None)
_outbound_start_child_workflow	_ChildWorkflowHandle	same pattern
_outbound_start_nexus_operation	_NexusOperationHandle[OutputT]	same pattern

Test plan

• Reproducer from Title: Python 3.14 Compatibility: NameError in _outbound_schedule_activity due to eager closure-cell inspection #1517 no longer raises NameError on Python 3.14
• Existing unit tests pass (python -m pytest tests/)
• No changes to public API or wire protocol

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