Limit page cache storage to successful HTML responses

[mehul0810]

Finding

The page-cache writer only skips empty output, responses with status code >= 400, and responses that set cookies. It does not require a 200/OK response and does not verify that the response is HTML before writing cache files.

Evidence

• src/Modules/Cache/PageCache.php store_cache() skips http_response_code() >= 400 but allows 3xx responses and other non-200 statuses.
• The same method checks for Set-Cookie headers but does not skip Location redirects or non-HTML Content-Type responses before writing metadata and body files.

Impact

Redirects or other non-HTML frontend responses can be captured as page-cache entries. Later visitors may receive stale or incorrect cached output without the original response headers, which can break redirects, plugin endpoints, or theme-level responses.

Suggested fix

Only write cache files for successful cacheable HTML responses. At minimum, require HTTP 200 and skip when Location headers are present. Prefer also checking response Content-Type when available and allowing only text/html.

Acceptance criteria

• 3xx responses are never written to the page cache.
• Non-HTML frontend responses are not cached.
• Normal public HTML pages still cache successfully.

[mehul0810]

Completed in PR #89 against release/1.6.0. Page-cache persistence is now limited to successful HTML responses.