Fix physics grid reparent crash

[metalgearsloth]

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Better solution would require the eventual SharedPhysicsMapComponent culling but this just fixes the debug assert in a slow way + adds a test to prevent it happening again.

Fixes https://github.com/space-wizards/RobustToolbox/issues/2989 as all of the physics re-parenting is tested (apart from awakebodies but that's going to get refactored eventually)