The previous test forced http_parser='python' to avoid a hard
dependency on gunicorn_h1c. Now run the same scenario under both
parser implementations so the smuggling guard is exercised on every
supported request-line/header path.
The smuggling guard added in #3614 reads self._body_receiver after
_handle_http_request returns to refuse keepalive on a body that did
not finish framing. But _handle_http_request's finally cleared the
receiver before returning, so the gate always saw None and let
keepalive proceed unconditionally. Move the clear into the
connection loop's per-iteration cleanup (it already had one there
for the same purpose).
Adds an end-to-end regression test that pipelines a partial-body POST
followed by a smuggled GET and asserts the second request is not
served and the transport closes.
