fix(serialization): accept non-string stored values in deserializeValue

[ralflang]

Summary

Reported on the horde mailing list: with Memcache configured, IMP dynamic view crashes on the ThemesCache read path:

```
TypeError: Horde\HashTable\Driver\Memcache::deserializeValue(): Argument #1 ($stored) must be of type string, int given, called in .../Driver/Memcache.php on line 99
and defined in .../SerializationTrait.php:57
```

Root cause

`HashTable\Driver\Memcache::getMultiple()` calls `$this->deserializeValue($val)` on every hit, where `$val` is what `MemcacheApi::getItems()` returns after its own `unserialize()` pass (see `MemcacheApi.php:583`).

The value in that slot is not always a string. Any of the following puts a non-prefixed value into the same memcached pool:

• The legacy `Horde_HashTable_Memcache` driver (still in `lib/`) writes raw values with no `s:`/`p:` prefix.
• The legacy `Horde_Cache_Storage_Hashtable::set()` branch (`lib/Horde/Cache/Storage/Hashtable.php:128`) stores `time()` as an int for the timestamp slot.
• Prior releases of this package running against the same pool before the upgrade.
• Any co-tenant component using `MemcacheApi` directly.

`SerializationTrait` already documents the intent to be forgiving ("Legacy data without prefix — treat as raw string", line 65–66), but the `string $stored` type declaration prevents that branch from ever running for non-strings and throws a TypeError instead.

Change

`SerializationTrait::deserializeValue()` now takes `mixed` and passes non-strings through unchanged. String values still go through the `s:`/`p:` prefix protocol; unprefixed strings still fall through to the documented raw-string path.

New unit tests in `test/src/Unit/SerializationTraitTest.php` cover:

• string / int / array round trips through `serialize`+`deserialize`
• non-string inputs (int, bool, null, array) pass through
• unprefixed strings return as raw
• `s:`-prefixed strings whose payload happens to start with `p:` are not misclassified

Fixes the mailing-list crash without changing behaviour for values written by the modern writer.