[moetemp]

What was changed

This PR adds the Activity Stream handles (ActivityStream, SyncActivityStream, SyncSubscription) and BufferedPublisher.

• The Activity's parameter type selects the handle class. A sync Activity gets the sync handles.
• An Activity writes as its attempt, so a retry withdraws the Records of the earlier attempts.
• BufferedPublisher batches the publishes of one Stream, in order, and flushes at the end of an Activity attempt.
• close on an Activity handle raises StreamNotOwnerError with no call, since no Activity owns a Stream. A Workflow's Stream closes from Workflow code, and a client's Stream from a client. The method stays, since the API proposal gives every handle the same calls.
• 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.