Accept a chunked upstream body of exactly the maximum size (closes #248)

Once upstream_max_response_size bytes were read, the fetcher refused the next read without looking for the end of the body. Go's HTTP client returns the end of a body sent with a Content-Length together with its last bytes, but the end of a chunked body can come in a later read, so a chunked body of exactly the limit was refused after it was downloaded whole and the image routes answered 500. Once the limit is used up, the fetcher now reads one more byte: if the body has ended it is accepted, and if there is more it is refused with ErrResponseTooLarge.

Model: opus-5-5

TODO.md

@@ -38,6 +38,15 @@ P2: security: per-origin rate limiting
   commented entry under `example.com`. Getting Started and "Running under upaas"
   in `README.md` say that every image on an allowlisted host is served without a
   signature, so an allowlisted host must be one the operator controls.
+- 2026-10-15 a chunked upstream body of exactly `upstream_max_response_size`
+  bytes is accepted (closes #248): once the limit was used up, the fetcher's
+  next read refused the body without looking for its end. Go's HTTP client
+  returns the end of a body sent with a `Content-Length` together with its last
+  bytes, but the end of a chunked body can come in a later read, so such a body
+  was refused after it had been downloaded whole, and the image routes
+  answered 500. Once the limit is used up, the fetcher now reads one more byte:
+  if the body has ended it is accepted, and if there is more it is refused with
+  `ErrResponseTooLarge`, as before.
 - 2026-10-13 `fit=outside` no longer makes an output above 8192 on a side
   (closes #257): it scales the image to cover the requested size, and the side
   scaled past that size was held to no maximum, so `1x8192?fit=outside` of a