[13.x] Make Cache touch() TTL required and remove redundant value fetching

[SanderMuller]

This PR makes two changes to Cache::touch() before the 13.x API surface is finalized:

1. Make $ttl a required parameter

Currently, Cache::touch($key) with no TTL silently converts the item to live forever via $this->forever().
This is surprising behavior for a method whose purpose is TTL extension. A developer calling Cache::touch('session-key') to "bump" an expiration would not expect to accidentally make the item permanent.

Unlike put(), where null TTL meaning "forever" is a convenience shorthand with a clear value being stored, touch() is specifically about managing expiration. Having it silently remove expiration contradicts its purpose.

Making $ttl required forces explicit intent: developers who want an item to live forever should call Cache::forever() directly.

2. Remove the redundant $this->get() call from the Repository layer

The current implementation fetches the full cached value at the Repository level before delegating to the store:

public function touch($key, $ttl = null)
{
    $value = $this->get($key);          // ← fetches full value
    if (is_null($value)) return false;
    // ...
    $this->store->touch($key, $seconds); // ← store doesn't need the value
}

This is unnecessary because every store implementation already handles the "key doesn't exist" case by returning false:

• Redis — EXPIRE returns 0 for nonexistent keys
• Database — UPDATE ... WHERE expiration > now affects 0 rows
• DynamoDB — ConditionalCheckFailedException caught and returns false
• Memcached — native touch() returns false
• File/APC — fetch the value internally and return false if missing

For the four atomic stores (Redis, Database, DynamoDB, Memcached), the Repository-level get() is pure overhead, it deserializes the entire cached value only to throw it away. For File and APC stores, the value ends up being fetched twice: once at the Repository layer and once inside the store.

After this change, the Repository simply converts the TTL and delegates:

public function touch($key, $ttl)
{
    return $this->store->touch($this->itemKey($key), $this->getSeconds($ttl));
}

Change before the new cache touch API goes live

This is a breaking change to a new cache API introduced in 13.x, so there is no backward-compatibility concern with 11.x or 12.x users. The method signature changes from touch($key, $ttl = null) to touch($key, $ttl).