Refactor ZipscriptMP3Handler and add JUnit test for invalid MP3s

[tsautier]

Problem

When encountering an invalid MP3 file, the slave would throw an IOException with message "X is not a valid MP3 file", causing errors in the async command handler.

Solution

Catch the specific IOException in ZipscriptMP3Handler.getMP3File() and return null instead of propagating the exception. This allows graceful handling of invalid MP3 files.

Changes Made

• Modified ZipscriptMP3Handler.java to catch IOException when parsing MP3 files
• If the error message indicates an invalid MP3 file, return null instead of throwing
• Other IOExceptions are still propagated

Fixes: #77

[tsautier]

@mvangoor @hrxcodes for review