Reject malformed chunk-size in chunked decoder

strtoul silently accepts a leading "-" and wraps via unsigned
arithmetic, so chunk-size "-2" produced ULONG_MAX-1, bypassing the
ULONG_MAX guard and letting a client drive the server toward unbounded
allocation.

Replace strtoul with a manual hex parser that requires at least one hex
digit, detects size_t overflow per digit, and accepts only chunk-ext or
end-of-line after the digits (RFC 9112 §7.1).