Fix for 6000.7.X: Rotation locked PhysicsBody incorrectly discards contact angular impulses resulting in linear over-correction which looks like an invalid bounce.

Issue:
A rotation-locked body kept its rotational inertia, so the contact solver split each impulse between linear and angular motion, then the lock discarded the angular part, leaving the linear velocity over-corrected into a small bounce; the fix zeroes the inertia while locked so impulses go entirely into linear motion.

Load the attached case submitted by the user here: https://discussions.unity.com/t/physics-core-2d-unexpected-bounce/1734698/8