Cache query AST as an array in store mode

[dxiiren]

Resolves #2782

• Added or updated tests
• Documented user facing changes
• Updated CHANGELOG.md (skip for docs-only changes)

Changes

query_cache.mode: store handed the DocumentNode straight to the cache store:

return $store->remember(key: "lighthouse:query:{$hash}", ttl: $this->ttl, callback: $parse);

Cache stores serialize what they are given. Laravel 13 lets an application limit which
classes come back out again through cache.serializable_classes, which every serializing
store reads before unserializing:

// Illuminate\Cache\RedisStore::unserialize(), same in DatabaseStore, FileStore, ArrayStore
if ($this->serializableClasses !== null) {
    return unserialize($value, ['allowed_classes' => $this->serializableClasses]);
}

Once that list is set and does not name every AST node class, the cached query comes back
as __PHP_Incomplete_Class and fromStoreOrParse() fails its return type on the second
request. Whitelisting the AST is not a practical workaround, since it would mean naming
every node class in webonyx/graphql-php and keeping that list current.

So this caches the array form instead of the object, which is what mode: opcache already
does via opcacheFileContents() and requireOPcacheFile(). Arrays are unaffected by
allowed_classes, so the AST survives regardless of how a store is configured.

The remember() call is replaced with the explicit get()/put() pair that
fromHybridOrParse() already uses, so anything found under the key that is not an array
is simply reparsed and overwritten. Caches populated by an older version therefore recover
by themselves on the next request, with no flush needed on upgrade.

Verified on PHP 8.4.20 / Laravel 13.23.0 / webonyx/graphql-php 15.37.1. The added test
fails on current master with exactly the reported error and passes with this change:

TypeError: Nuwave\Lighthouse\Cache\QueryCache::fromStoreOrParse(): Return value must be
of type GraphQL\Language\AST\DocumentNode, __PHP_Incomplete_Class returned
src/Cache/QueryCache.php:84
src/Cache/QueryCache.php:72
src/GraphQL.php:287

It reproduces through the array store with serialize enabled, so it needs no Redis or
database. The rest of QueryCacheTest is unchanged by this commit.

Breaking changes

None. The cache key is unchanged, entries written by older versions are detected and
reparsed rather than failing, and the public API is untouched.