PERCENTAGE_DAMAGE spells use the right bodypart's hp

[Procyonae]

Summary

None

Purpose of change

Fixes #57875

Describe the solution

Uses the same function as the eventual perfectly aimed hit does to ensure it works even on non humanoid body creatures.

Describe alternatives you've considered

Just using { body_part_torso }

Testing

Tested using a lvl 20 disruption bolt with "valid_targets": [ "self" ],

Additional context