Response header detection in page cache test for Site Health should be more robust

[westonruter]

This is a follow-up to #63748. See ​PR comment from @dmsnell:

I wish that the $cache_hit_callback were more robust than it is, since it also matches on values like “this cache is *hit, don’t use it” and “not a hit” but that’s not part of this ticket or work.

would be awesome to have some example strings from each of these new headers as a comment to the right of them.

for example, the ​varnish docs suggest that the _full_ match is hit, meaning we could add => static function ( $v ) { return 'hit' === $v; } /** @see https://www.varnish-software.com/developers/tutorials/logging-cache-hits-misses-varnish/ */

the extra examples are icing on the cake and not necessary here.

according to ​this random survey x-cache-status is expected to only contain hit, though ​for x-cache there is an insignificant but measureable count of requests containing HIT, MISS

Also, ​in Slack:

the only thing that really caught my eye is that we’re needly allocating to do case-insensitive compare instead of calling stripos()

[solankisoftware]

FIX :: Response header detection in page cache test for Site Health should be more robust

[solankisoftware]

Replaces the old string-based cache header detection in Site Health with a more robust method.
Adds case-insensitive matching, supports additional cache headers, and prevents false positives.
Improves accuracy across Varnish, NGINX, Cloudflare, LiteSpeed, and host-level caching systems.

[westonruter]

@solankisoftware Your patch seems malformed. It is introducing an entire new file. Please consider opening a pull request instead to facilitate review and testing.

Trac Ticket: https://core.trac.wordpress.org/ticket/64370

This PR enhances the WordPress Site Health page cache test by introducing
a robust cache header detection mechanism. The previous method could
produce false positives or negatives depending on the caching system used.

Changes include:

• Added detect_cache_headers() method for strict, case-insensitive header matching
• Improved support for common cache headers (x-cache, x-cache-status, cf-cache-status, x-litespeed-cache, x-varnish, etc.)
• Updated get_test_page_cache() to use detect_cache_headers()
• Prevents false positives when unrelated text contains words like "HIT" or "MISS"

Testing:

1. Enable any caching layer (Varnish, NGINX fastcgi_cache, Cloudflare, LiteSpeed, host-level caching)
2. Go to Tools → Site Health → Status
3. Verify that page cache detection reflects the real server cache status

This PR is backward compatible and does not change existing Site Health UI behavior.