/h/{uuid} has no MaxBodySize middleware, unlike the page route
groups; its cap is in the handler, which owns the 413 senders get.
A comment at the route registration now says so, and the handler's
read function notes it is the receiver's only body cap.
A new routing test sends a body exactly at the cap and one byte over
it through the production router, and checks the second is refused
with the handler's 413 and message.
Model: opus-5-5