Parses a URL-encoded request body, checking each pair as it streams and charging budget (the request's own budget by default).

A charset declared in the Content-Type is decoded to UTF-8 while the body streams, as HTTP::Request#form_params does in Crystal.

Amber parses the form before the action runs (the CSRF pipe reads _csrf from params), which drains the body. The bytes the parse reads are kept and put back into request.body, so the action reads exactly what the client sent. The parse never reads more than max_form_body_bytes, which bounds the kept copy; a body whose declared Content-Length is already over that limit is rejected without keeping anything.