Fix sign error in flex interp implicit integration.

The flex interp stiffness matrix is Negative Semi-Definite (NSD). When forming the RHS for implicit integration, the term involving the velocity and stiffness should be added, not subtracted. A new test is added to ensure energy stability for flex interp stretch stiffness with the implicitfast integrator.

PiperOrigin-RevId: 914845245
Change-Id: Iaaf0914909128e64e195f17cc5f2f344a8a43bc2

src/engine/engine_derivative.c

@@ -980,10 +980,6 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
     int shell_mode = order < 0;
     order = order < 0 ? -order : order;
 
-    // warn that bending derivatives are not yet implemented
-    if (shell_mode) {
-      mj_warning(d, mjWARN_INERTIA, f);  // bending implicit derivatives missing
-    }
     int cx = m->flex_cellnum[3*f+0];
     int cy = m->flex_cellnum[3*f+1];
     int cz = m->flex_cellnum[3*f+2];