Simplify FileCacheStorage::save() with atomic dumpFile()

[TomasVotruba]

What

Simplifies FileCacheStorage::save() to write the cache file with a single atomic Filesystem::dumpFile() call, removing the manual temp-file handling.

Why

This finishes the cleanup started in #498 ("Sync FileCacheStorage with changes from phpstan-src"), which had since regressed. The write path currently does its own atomic-write dance:

$tmpPath = sprintf('%s/%s.tmp', $this->directory, Random::generate());
FileSystem::write($tmpPath, ...);   // Nette
$copySuccess = @copy($tmpPath, $filePath);
@unlink($tmpPath);
// + a DIRECTORY_SEPARATOR fallback

But the already-injected Symfony Filesystem::dumpFile() already writes atomically via a temporary file + rename internally, so doing it again on the Rector side is redundant syscalls — the exact overhead #498 set out to remove (it was a notable bottleneck on Windows).

How

• Replaced the Random temp path + FileSystem::write + @copy + @unlink + DIRECTORY_SEPARATOR fallback with $this->filesystem->dumpFile($filePath, ...).
• Kept the var_export error guard (catches non-exportable data such as closures).
• Dropped the now-unused Nette\Utils\Random import.

Behaviour is unchanged; this is a smaller/faster write path.

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Generated by Claude Code