Prevent workflow stream subscriptions from hiding cancellation

[1fanwang]

When an activity blocks in WorkflowStreamClient.subscribe and its workflow
cancels it, the subscription currently ends like a normal iterator. The activity
can then return success even though Temporal requested cancellation. This change
propagates the cancellation to the activity.

The subscription now rethrows direct task cancellation and cancellation wrapped
by WorkflowUpdateRPCTimeoutOrCancelledError. RPC timeouts and server-side RPC
cancellation keep their existing continue-as-new and terminal-workflow handling.

Testing

The regression test starts a real Temporal dev server, waits until a heartbeating
activity is polling a workflow stream, and then cancels that activity. The
activity catches CancelledError directly around the subscription and returns a
distinct sentinel, so an error added later by the activity runtime cannot satisfy
the assertion.

Before

$ git checkout upstream/main -- temporalio/contrib/workflow_streams/_client.py
$ uv run pytest tests/contrib/workflow_streams/test_workflow_streams.py::test_activity_subscription_propagates_cancellation -q --no-header
E           AssertionError: assert 'subscription-ended' == 'subscription-cancelled'
Results (3.30s):
         1 failed

After

$ git checkout HEAD -- temporalio/contrib/workflow_streams/_client.py
$ uv run pytest tests/contrib/workflow_streams/test_workflow_streams.py::test_activity_subscription_propagates_cancellation -q --no-header
.                                                                        [100%]
Results (2.95s):
         1 passed