fix(task): accept any 2xx upstream status on task submission

The submit relay treated every status other than 200 as an upstream
failure, so a task API that answers 201 Created or 202 Accepted (for
example Tencent's wand-vega-video-1.0-lite) returned fail_to_fetch_task
and parseSubmitResponse never ran. The check predates the plugin system,
when every built-in adaptor answered 200; the plugin adaptor already
passes the exact statusCode to parseSubmitResponse and polling has
always classified 2xx as a successful response.

Accept any 2xx before parsing and keep failing other statuses with the
upstream status and body. Add a relay regression that drives a plugin
submission against a real upstream returning 200/201/202 and 400/502,
and document the submit-side 2xx contract in the plugin API.

Fixes #7478