[moetemp]

What was changed

This PR adds the Stream calls in Activities, and BufferedPublisher, SyncStream and SyncSubscription in temporalio.client.

• In an Activity, a Stream of the Worker's client, such as from activity.client().to_stream(ref), writes as the attempt, with the activity or standalone_activity epoch case. So a retry withdraws the Records of the earlier attempts, even if it writes nothing. A Stream of another client writes as that client.
• A sync Activity has no activity.client(), so it keeps the Worker's client, such as in an Activity class, and wraps a Stream in SyncStream(stream), whose calls block. publish and subscribe of a Stream in a sync Activity raise StreamUnsupportedError, and the message points to SyncStream.
• publish starts the write at the call. The end of the attempt waits for the attempt's writes, and a write error that no caller got fails the attempt.
• BufferedPublisher(stream, *, max_delay=...) takes a Stream and batches its publishes, in order, and flushes at the end of an Activity attempt. Workflow code has no Stream, and a WorkflowStream passed with no type checks raises StreamUnsupportedError.
• An Activity never closes a Stream. close, async with and SyncStream.close raise StreamNotOwnerError with no call, and async with raises at __aenter__. producer raises StreamUnsupportedError, since the attempt is the Producer.
• A sync call runs on the Worker's loop. run_blocking makes the call's future before it schedules the call, so a cancel of the Activity thread after the schedule also cancels the call. A call when the Worker's loop is already closed closes its coroutine, so nothing warns at shutdown.

Why?

Agents stream model tokens from an Activity. The Workflow keeps the Stream, and the Activity writes to it.

Part of the Streams review series.

Checklist

1. How was this tested:
   • poe lint and pytest tests/contrib/streams on both backends.

[moetemp]

Closed in favor of #381, the combined branch, which has this change.