Disable island discovery for models with flexes.

PiperOrigin-RevId: 574244981
Change-Id: I7c08b23cf5f29338a04ec684ad7a0d4d8a0397df

src/engine/engine_forward.c

@@ -619,6 +619,7 @@ void mj_fwdConstraint(const mjModel* m, mjData* d) {
 
   // check if islands are supported
   int islands_supported = mjENABLED(mjENBL_ISLAND)  &&
+                          d->nisland > 0            &&
                           m->opt.solver == mjSOL_CG &&
                           m->opt.noslip_iterations == 0;
 

src/engine/engine_island.c

@@ -407,7 +407,7 @@ void mj_island(const mjModel* m, mjData* d) {
   int nv = m->nv, nefc = d->nefc, ntree=m->ntree;
 
   // no constraints: quick return
-  if (!nefc) {
+  if (!nefc || m->nflex) {  // TODO: add flex support to island discovery
     d->nisland = 0;
     return;
   }