Render zero-radius round cuboids as cuboids in debug output

[fyrsta7]

Summary

Fixes #969 by rendering zero-radius round cuboids through the regular cuboid debug-render path.

Details

A RoundCuboid with a zero border radius is geometrically equivalent to its inner cuboid. The rounded-cuboid outline generation path can fail for that degenerate radius, so the debug renderer now handles this case directly for both 2D and 3D.

Validation

• git diff --check
• cargo fmt --all --check
• cargo test -p rapier3d --features debug-render debug_render_round_cuboid_with_zero_radius
• cargo check -p rapier2d --features debug-render

[fyrsta7]

Closing this draft because the underlying issue was fixed by dimforge/parry#437, which was merged after this PR was opened. The Rapier issue is now resolved, so this change is no longer needed.