fix: update cache on validation

[cih9088]

When the cache is stale, it fails to freshen the headers upon receiving a 304 response, resulting in re-validation for every subsequent request.

From https://www.rfc-editor.org/rfc/rfc9111.html#section-4.3.4

For each stored response identified, the cache MUST update its header fields with the header fields provided in the 304 (Not Modified) response, as per Section 3.2.

Dismissing my approval since the affected tests need to be updated to match the new logic. See my review comment for details.

The lint workflow failure is just pointing out some outdated file headers. I'll fix those before the next release.