[codex] Tests: cover client_max_body_size on chunked discard path

[maxvanamersfort]

Summary

Add regression coverage for nginx issue #1522 and companion source PR
nginx/nginx#1688.

The tests cover a discarded chunked request body:

• exactly at client_max_body_size across multiple chunks;
• over the limit in one chunk;
• over the limit cumulatively across multiple chunks.

Red-to-green result

Against nginx baseline 3f6f7824d4e2eb1ac37dec76683d525ac0ff521c, the
two over-limit assertions fail because nginx returns 200 instead of 413.

Against the companion source patch, all 23 assertions in body_chunked.t
pass. The broader body.t, body_chunked.t, and http_keepalive.t run also
passes all 63 tests, with two pre-existing TODO assertions remaining TODO.

Related issue: nginx/nginx#1522