Prevent workflow stream subscriptions from hiding cancellation

[1fanwang]

What was changed

An activity whose workflow stream subscription is cancelled now raises
CancelledError instead of seeing the iterator end normally. Before this change,
such an activity could return success even though Temporal had requested its
cancellation.

The subscription 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.

Why?

Activity code that loops over a subscription has no other way to learn that
Temporal cancelled it, so cleanup and cancellation reporting were skipped.

Checklist

1. Closes: no linked issue.

2. How was this tested:

The probe starts a local Temporal dev server, runs the regression test's
workflow and activity, waits until the heartbeating activity is polling a
workflow stream, and then cancels it. The activity catches CancelledError
directly around the subscription and returns a distinct value for each outcome.

Reproducer source: probe.py

import asyncio

from temporalio.testing import WorkflowEnvironment

from tests.contrib.workflow_streams.test_workflow_streams import (
    CancellableSubscriber,
    CancelSubscriptionWorkflow,
)
from tests.helpers import new_worker


async def main(timeout: float) -> None:
    async with await WorkflowEnvironment.start_local() as env:
        subscriber = CancellableSubscriber()
        async with new_worker(
            env.client, CancelSubscriptionWorkflow, activities=[subscriber.subscribe]
        ) as worker:
            handle = await env.client.start_workflow(
                CancelSubscriptionWorkflow.run,
                id="workflow-stream-cancel-probe",
                task_queue=worker.task_queue,
            )
            await asyncio.wait_for(subscriber.started.wait(), timeout)
            await handle.signal(CancelSubscriptionWorkflow.cancel_subscription)
            print("activity result:", await handle.result())


asyncio.run(main(timeout=10))

Before, with the stream client from main:

$ uv run python probe.py
activity result: subscription-ended

After, with this branch:

$ uv run python probe.py
activity result: subscription-cancelled

3. Any docs updates needed?

No. The CHANGELOG entry is under Unreleased.

[brianstrauch]

Thank you!

[brianstrauch]

@1fanwang From CI:

Error: CHANGELOG.md additions must be under the Unreleased section.

[1fanwang]

CHANGELOG.md additions must be under the Unreleased section.

@brianstrauch Done in 55d19f7.