fix: handle bytes output when commands time out

[harshitethic]

Problem

subprocess.TimeoutExpired.stdout may be bytes even when the subprocess was started in text mode. The timeout handler concatenated that value directly with a string, so a timed-out command that had already produced output could raise TypeError instead of returning PatchPilot's normal timeout result.

Fix

• normalize captured timeout output to text before appending the timeout marker
• preserve the existing return code 124
• add a focused regression test using byte output from TimeoutExpired

This keeps timeout failures reportable instead of turning them into backend errors.