[6.x] Fix stale static cache status on responses in long-lived processes

[petemolinero]

Static caching keeps a small lookup table that remembers, for each response, whether it was served from the static cache. That table is keyed by the response's PHP object id, and entries are never removed.

In a normal PHP-FPM request that's harmless: the whole process ends with the request, and the table dies with it. In a long-lived process (e.g. Octane, queue workers) the same PHP process handles many requests, so two things go wrong:

• Stale answers. PHP recycles object ids once an object is gone. A new response can be handed the id of an old one and inherit its cached status, so wasStaticallyCached() can say "yes" for a response that was never cached.
• A table that only grows. Every response adds an entry that is never cleaned up.

The fix

Key the table by the response object itself using PHP's WeakMap (added in PHP 8.0), which is a map whose entries are dropped automatically as soon as the object they belong to is gone. No stale ids, no cleanup to remember, and the map never grows past the responses that are still alive. No public API changes: the setStaticCacheResponseStatus, staticCacheResponseStatus, and wasStaticallyCached macros behave exactly as before.