[moetemp]

What changed?

This PR adds @nexus.poll to the Python client target. An operation with @nexus.poll python="<helper>" targets="client" gets an iter_<name> async iterator. It yields each response and asks the support package's <helper>(request, response) for the next request, which returns None to stop.

• The client extension gets iter_<name>, and so does each client resource method that binds the operation. The stream sample gets iter_read_records and Stream.iter_read. Its helper ends a read that doesn't wait at the newest Record, and starts each later read at the last page's next_offset.
• Each page is its own operation, with its own random operation ID. The extension iterator uses one Nexus client for all pages.
• nexgen refuses the directive with no targets, with targets that include workflow, on a resource method, and on an operation it can't poll. The Workflow package doesn't change.

Why?

A WIT stream can't cross a Nexus call, so the Stream read is a request and a response. The iterator reads every page, and the support package decides the next request.

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

test_stream_client.py reads every page through client.iter_read_records and through Stream.iter_read, ends a read that doesn't wait at the newest Record, and starts the read after start=END at the first page's offset. Rejection fixtures cover the client-only rule, an operation that returns a resource, and an iterator name clash.