* requestbody: reject oversized Content-Length before the next handler
When max_size is set and the request declares a Content-Length above it, respond 413 right away instead of letting the next handler start and fail mid-read. Bodies of unknown length are still limited by MaxBytesReader. Both paths return the same error, so handle_errors sees no difference.
* caddytest: send unknown-length bodies in max_size placeholder tests
request_body now answers 413 for a declared Content-Length over max_size before the placeholder reads the body, so these tests no longer covered the read path and the direct respond one failed.
* requestbody: keep the body limited for error routes
Error routes get the same request, so wrap the body before the early 413 to stop them from reading past max_size.
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