Answer 415 for a source whose first bytes do not match its Content-Type (closes #253)

A body whose first bytes are not those of the format its Content-Type
names answered 500 on both image routes, was logged as pixa's own
error, and was fetched again on every request. Both routes now answer
415 for each error the first-bytes check returns, and the image service
logs the refused source at warning level. The failure also gets a
negative cache entry, as an upstream error status does, so the source
URL is not fetched again for 5 minutes, and meanwhile an image that
would need that fetch answers 502. README.md says so.

The case of TestService_Get_ConcurrentMissesShareAFailure that expected
such a source to be fetched again now expects the negative cache, as
#280 (comment) allows.

Model: opus-5-5

TODO.md

@@ -30,6 +30,15 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-20 a source whose first bytes do not match its `Content-Type` answers
+  415 (closes #253): a body whose first bytes are not those of the format its
+  `Content-Type` names, such as a JPEG sent as `image/png`, text, or a body
+  under 12 bytes, answered 500 on both image routes and was logged as an error
+  of pixa's, and each request for it fetched the whole body again. Both routes
+  now answer 415, and the image service logs it at warning level as a refused
+  source. It also gets a negative cache entry, as an upstream error status does,
+  so the source URL is not fetched again for 5 minutes, and meanwhile an image
+  that would need that fetch answers 502.
 - 2026-10-20 `AGENTS.md`: a bug fix may change an existing test without asking
   when that test pins exactly the behavior the filed bug calls wrong, and the
   change moves it to the fixed behavior and nothing else (closes #280).