fix(runtime): enforce request limits before body buffering

[doublewhy]

Problem

The control-plane request-size limit runs after framework buffering. A chunked request can use unbounded memory before rejection, so API-404 cannot claim bounded ingress.

Done when

• Count delivered bytes at the public ASGI receive boundary before application buffering; reject overflow with a stable HTTP 413 response.
• The limit works without Content-Length, with a false length, and with chunked bodies under, at, or over the limit.
• Tests cover many small chunks, zero-length bodies, disconnects, unchanged normal streaming, and that the endpoint is never called after overflow.
• Docs state bounded routes, configuration, counting point, 413 response, and proxy expectations.

Avoid private Starlette body state and NOSONAR suppression. Parent: #8. Current implementation: #1086.