fix: Guard against race condition in FileStore::createCacheDir

[Sarsharses]

Problem

Under concurrent uploads, FileStore::createCacheDir() can crash with mkdir(): File exists. On PHP setups that promote warnings to exceptions (e.g. Laravel's error handler), this surfaces as a fatal ErrorException on the first upload after the cache directory is absent.

Root cause

The check-then-create is not atomic:

if ( ! file_exists($this->cacheDir)) {
    mkdir($this->cacheDir);
}

Two concurrent requests can both pass file_exists(), then both call mkdir(); the loser gets a File exists warning.

Note: this is a different race from the cache-file data race fixed previously (#366, #368). Those hardened reads/writes of the cache file, whereas this fixes creation of the cache directory itself.

Fix

• Suppress the race-loser warning with @mkdir(); the directory ends up existing either way.
• Use is_dir() (the correct predicate, since file_exists() is also true for a file at that path).
• Create recursively (0777, true) so nested cache paths work; default mode is unchanged.

Testing

Added it_creates_nested_cache_dir_recursively, which fails on main and passes with the fix. Full suite stays green.

A pure concurrency race can't be reproduced deterministically in a single-threaded test; the added test covers the recursive-creation half, and the warning suppression is the standard idiom used by Symfony/Laravel mkdir helpers.