Previously, for regular handlers the request body was read in routing()
before the route was matched, so the pre-request handler always saw an
already-read body. The ContentReader path, in contrast, ran the
pre-request handler before the body was read. This inconsistency made
it impossible to reject a request (e.g. failed per-route authentication
via req.matched_route) without buffering a potentially large body.
Move the read_content() call into dispatch_request(), after route
matching and the pre-request handler, so both paths behave the same:
route matching -> pre_request_handler -> body read -> handler. A
request rejected by the pre-request handler no longer reads the body
at all; the existing keep-alive drain logic still consumes any framed
body afterwards.
Note: code that referenced req.body or body-derived form fields inside
the pre-request handler will now see an empty body. Inspect headers,
path, query parameters, or matched_route instead.
Also document the handler execution order in README and update the
pre-request cookbook pages (en/ja).
