Clean up temporary files created for Windows background processes

[Copilot]

The background_proc() method creates temporary files for stdout/stderr on Windows but never deletes them, leaving orphaned files on the filesystem.

Changes:

• Track temp files: Added $temp_files property to store file paths for cleanup
• Clean on error: Delete temp files immediately when process fails to start
• Clean on success: Delete temp files in AfterScenario hook when process terminates
• Helper method: Extracted cleanup_temp_files() to handle safe deletion with existence checks

Implementation:

// Create and track temp files
$stdout_file = tempnam( sys_get_temp_dir(), 'behat-stdout-' );
$stderr_file = tempnam( sys_get_temp_dir(), 'behat-stderr-' );
// ...
$this->temp_files[] = $stdout_file;
$this->temp_files[] = $stderr_file;

// Cleanup helper with existence check
private function cleanup_temp_files( ...$files ): void {
    foreach ( $files as $file ) {
        if ( file_exists( $file ) ) {
            unlink( $file );
        }
    }
}

Temp files now cleaned up regardless of process success/failure.

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[Copilot]

Pull request overview

This PR fixes a resource leak where temporary files created for background processes on Windows were never cleaned up. The implementation adds proper tracking and cleanup of these temporary files both on process failure and at scenario completion.

Changes:

• Added $temp_files property to track temporary file paths for cleanup
• Implemented cleanup logic in afterScenario hook after process termination
• Added immediate cleanup when background process fails to start on Windows

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