[MoM] Add EoC activity check for stopping learning power meditation

[Standing-Storm]

Summary

Bugfixes "[MoM] Add EoC activity check for stopping learning power meditation"

Purpose of change

Because there's no u_has_activity condition, I have to use other safeguards and workarounds to make sure people don't start the meditation to learn powers, stop them and run off to do other things, and still get their powers anyway. It looks like it was still happening, so time to add another safeguard.

Describe the solution

Use a character_finishes_activity EoC to check if the meditation was manually interrupted and remove the effect that the success/fail EoC relies on to fire, preventing it from activating.

Describe alternatives you've considered

Testing

Interrupted meditation, effect successfully removed.

Additional context