[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(handle), writes as the attempt. So a retry withdraws the Records of the earlier attempts. 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 names 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.
• close, async with and SyncStream.close raise StreamNotOwnerError in an Activity with no call, and async with raises at __aenter__. Only the creator's side closes a Stream, so a Workflow's Stream closes from Workflow code and a client's Stream from a client. The methods stay, since Stream is also the client's class. producer raises StreamUnsupportedError, since the attempt is the Producer.
• A sync call runs on the Worker's loop. _run makes the call's future before it schedules the call, so a cancel of the Activity thread after the schedule also cancels the call. With asyncio.run_coroutine_threadsafe, a cancel can leave a long poll running. A call when the Worker's loop is already closed closes its coroutine, so nothing warns at shutdown.

Depends on S7 (#377). No changelog fragment, since S9 has the Streams entry.
Part of the Brook review series.

Why?

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

Checklist

1. How was this tested:
   • poe lint, poe gen-docs and pytest tests/contrib/streams on both backends. The sync cancel tests ran in a loop on Python 3.13 and 3.14, alone and in parallel.
   • Fork CI passes, except the arm jobs, since the fork has no arm runners.