Introduces a new "strain" equality mode (flex_edgeequality = 3) for flexcomps with trilinear interpolation. This mode enforces constraints on the three invariants of the Green-Lagrange strain tensor at 8 Gauss quadrature points within each flex element, resulting in 24 equality constraints per flexcomp.

Also fixes a bug when attaching a flex with constraints.

PiperOrigin-RevId: 881349765
Change-Id: I8e6df8239488bfdc87d7e5b473568975e38c737e

unity/Runtime/Bindings/MjBindings.cs

@@ -288,7 +288,8 @@ public enum mjtEq : int{
   mjEQ_TENDON = 3,
   mjEQ_FLEX = 4,
   mjEQ_FLEXVERT = 5,
-  mjEQ_DISTANCE = 6,
+  mjEQ_FLEXSTRAIN = 6,
+  mjEQ_DISTANCE = 7,
 }
 public enum mjtWrap : int{
   mjWRAP_NONE = 0,