Improve Site Health Page Cache Detection Logic

[solankisoftware89]

Trac Ticket: https://core.trac.wordpress.org/ticket/64366

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.

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