[sajjadhossainshohag]

Cache::memo()->tags()->many() returns null for cached values when the keys are integers, such as model ids:

Cache::tags(['users'])->put(10, 'Taylor', 60);
Cache::tags(['users'])->put(20, 'Tim', 60);

Cache::tags(['users'])->many([10, 20]);         // [10 => 'Taylor', 20 => 'Tim']
Cache::memo()->tags(['users'])->many([10, 20]); // [10 => null, 20 => null]

The same happens through get([...]) and getMultiple(), and when some of the keys were already memoized.

MemoizedTaggedCache::many() combines the memoized values with the ones it just retrieved using array_merge(), which renumbers integer keys. 10 and 20 become 0 and 1, so the array_key_exists() lookup that builds the result misses and falls back to the default.

This is the same bug that #57862 fixed for the untagged MemoizedStore, and the tagged memoized cache from #61593 brought it back. This PR uses += instead, which keeps the keys. The two arrays never share a key, because only missing keys are retrieved.

The tests cover a fresh retrieval, a second memoized retrieval, and a partially memoized one.