fix(Auth): retain expiration in FileSystemCacheItemPool and return null in CacheTrait

[cy-yun]

Fixes #9669

Problem

In FileSystemCacheItemPool::save(), the pool called serialize($item->get()). This discarded all expiration metadata and cache item status on save. When getItem() was subsequently called, it instantiated a new TypedItem($key) and called $item->set(...), setting $isHit to true and clearing any expiration. As a result, items never expired in the file cache, leading to stale authentication tokens being reused indefinitely. Additionally, in CacheTrait::getCachedValue(), when $cacheItem->isHit() was false, the method implicitly fell off without an explicit return statement.

Solution

1. FileSystemCacheItemPool::save(): Directly serialize $item (serialize($item)) so expiration and cache metadata are retained.
2. FileSystemCacheItemPool::getItem(): Unserialize the data; if it is an instance of CacheItemInterface, return it directly. For backward compatibility with legacy cache files on disk containing raw serialized values, fallback to creating a new TypedItem($key) and setting the value.
3. CacheTrait::getCachedValue(): Add an explicit return null; at the end of the method when cache items are missed or expired.

Verification

• Added testSaveAndGetExpiredItem in FileSystemCacheItemPoolTest verifying that expired cache items return false for isHit() and null for get().
• Added testGetItemLegacyCacheFile in FileSystemCacheItemPoolTest verifying backward compatibility with legacy serialized cache files.
• Added testFailsPullFromCacheWhenItemIsNotHit in CacheTraitTest verifying explicit null return on cache miss.
• Ran test suite: HOME=/tmp vendor/bin/phpunit Auth/tests/Cache/FileSystemCacheItemPoolTest.php (passed).
• Ran test suite: HOME=/tmp vendor/bin/phpunit Auth/tests/CacheTraitTest.php (passed).
• Ran CS Fixer: vendor/bin/php-tools cs-fixer googleapis/google-cloud-php --ref HEAD (0 files that can be fixed).