fix: use default permissions for test temporary directories

Changed mkdir() call to use default permissions instead of 0777.
The 0777 mode made directories world-writable, which is a security
concern. Now using system umask defaults (typically 0755 or 0700).

Author: claude

tests/src/Functional/SplitCommandTest.php

@@ -21,7 +21,7 @@ protected function setUp(): void
 
         // Create a temporary directory for test outputs
         $this->tempDir = sys_get_temp_dir() . '/har_test_' . uniqid();
-        mkdir($this->tempDir, 0777, true);
+        mkdir($this->tempDir, recursive: true);
 
         // Set up the command tester
         $command = new SplitCommand();