Include gap in body_margin, fixing mid-phase pruning of in-gap contacts.

The mid-phase BVH descent filter prunes body pairs using body_margin,
which was compiled as the max over geom margins, excluding gap. Broadphase
and the leaf-level test both use margin+gap, so any multi-geom body relying
on gap could silently lose its in-gap contacts when raw AABBs don't
overlap. Single-geom bodies take the leaf-leaf path and were unaffected.

PiperOrigin-RevId: 946967548
Change-Id: I6d92baa296f1a83be68b4dbfd64a96d1c7efd3c4

python/mujoco/introspect/structs.py

@@ -1612,7 +1612,7 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=PointerType(
                      inner_type=ValueType(name='mjtNum'),
                  ),
-                 doc='MAX over all geom margins',
+                 doc='MAX over all geom margins+gaps',
                  array_extent=('nbody',),
              ),
              StructFieldDecl(