Improve mj_addM and associated test

- Updated docs to clarify that only the lower triangle is touched
- The function was unnecessarily adding the upper triangle in the sparse case
- The test used an unnecessarily large model and stepping it leading to timeouts under ASAN

PiperOrigin-RevId: 798528736
Change-Id: I592cc17dfb62379bb0cd6b332a8eb7e8b0ef6355

src/engine/engine_support.c

@@ -1038,7 +1038,7 @@ void mj_addM(const mjModel* m, mjData* d, mjtNum* dst,
 
   // dense
   else {
-    mju_addToSymSparse(dst, d->M, nv, m->M_rownnz, m->M_rowadr, m->M_colind, /*flg_upper=*/ 1);
+    mju_addToSymSparse(dst, d->M, nv, m->M_rownnz, m->M_rowadr, m->M_colind, /*flg_upper*/ 0);
   }
 }