Fix circular import: do not import temporalio.client in temporalio.nexus

[dandavison]

On main, a circular import error is revealed by doing e.g.

uv run tests/nexus/test_type_checking.py


Traceback (most recent call last):
  File "/Users/dan/src/temporalio/sdk-python/tests/nexus/test_type_checking.py", line 3, in <module>
    import temporalio.nexus
  File "/Users/dan/src/temporalio/sdk-python/temporalio/nexus/__init__.py", line 9, in <module>
    from ._decorators import workflow_run_operation as workflow_run_operation
  File "/Users/dan/src/temporalio/sdk-python/temporalio/nexus/_decorators.py", line 19, in <module>
    from temporalio.nexus._operation_context import (
  File "/Users/dan/src/temporalio/sdk-python/temporalio/nexus/_operation_context.py", line 22, in <module>
    import temporalio.client
  File "/Users/dan/src/temporalio/sdk-python/temporalio/client.py", line 60, in <module>
    import temporalio.workflow
  File "/Users/dan/src/temporalio/sdk-python/temporalio/workflow.py", line 62, in <module>
    from temporalio.nexus._util import ServiceHandlerT
  File "/Users/dan/src/temporalio/sdk-python/temporalio/nexus/_util.py", line 21, in <module>
    from temporalio.nexus._operation_context import WorkflowRunOperationContext
ImportError: cannot import name 'WorkflowRunOperationContext' from partially initialized module 'temporalio.nexus._operation_context' (most likely due to a circular import) (/Users/dan/src/temporalio/sdk-python/temporalio/nexus/_operation_context.py)

While that's not a command a user would actually run, the situation it reflects is undesirable and this will bite us at some point: temporalio.nexus has a run-time import dependency on temporalio.client.

This PR removes the dependency.

[tconley1428]

Why is client still needed in type checking if the callback has moved here? Is there another thing?