fix: avoid spurious "exception in shielded future" logs on cancellation

[Bathula-Adiseshu]

What was changed

Replaced internal asyncio.shield() usage in workflow outbound cancellation loops with a custom _shield_await() helper.

The helper uses asyncio.wait() to preserve the behavior we need:

• the underlying future is not cancelled when the waiting task is cancelled
• the future result/exception is still propagated correctly
• avoids the internal asyncio.shield() exception logging callback that caused spurious ERROR logs

Added regression coverage to verify cancelling workflows with in-flight activities does not emit "exception in shielded future" ERROR logs.

Why?

On Python 3.11+, asyncio.shield() can attach an internal exception logging callback when the outer task is cancelled.

Temporal workflow cancellation handling intentionally catches CancelledError, clears cancellation state, and waits for the operation to finish. In this flow, the original shield callback could remain attached to the future. When the activity later completed with an ActivityError due to cancellation, the workflow handled it correctly, but the leftover callback produced an orphan ERROR log.

This change removes that behavior while keeping the cancellation semantics required by Temporal.

Checklist

1. Closes [Bug] Single spurious "exception in shielded future" ERROR log per cancelled in-flight activity on Python 3.11+ (residual after #1523) #1600

2. How was this tested:

• Added regression test:
  • test_workflow_cancel_no_shielded_future_log
• Verified with:
  • poe lint
  • pytest tests/worker/test_workflow.py::test_workflow_cancel_no_shielded_future_log
  • pytest tests/worker/test_workflow.py

3. Any docs updates needed?

No docs updates needed.

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