Async generator bodies don't execute until __anext__(), by which time
the request context is gone. Restructure execute_page_streaming as a
regular async function that does all context-dependent work (g, request,
current_app access, layout resolution, task creation) while the context
is live, then returns an inner async generator that only yields strings
and awaits pre-created tasks.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>