Add mutation testing

[imsus]

Description

Add mutation testing to validate test quality and catch weak tests.

File to modify:

• composer.json

Change:

Add dev dependency:

"pestphp/pest-plugin-mutate": "^1.0"

Benefits:

• Detects tests that don't actually validate anything
• Improves test suite reliability
• Catches redundant or useless assertions

Commands to add in composer.json:

"mutate": "pest --mutate",

Priority: P4 (Optional, High impact, Medium effort)