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

include/mujoco/mujoco.h

@@ -512,7 +512,7 @@ MJAPI void mj_mulM(const mjModel* m, const mjData* d, mjtNum* res, const mjtNum*
 // Multiply vector by (inertia matrix)^(1/2).
 MJAPI void mj_mulM2(const mjModel* m, const mjData* d, mjtNum* res, const mjtNum* vec);
 
-// Add inertia matrix to destination matrix.
+// Add inertia matrix to destination matrix (lower triangle only).
 // Destination can be sparse or dense when all int* are NULL.
 // Nullable: rownnz, rowadr, colind
 MJAPI void mj_addM(const mjModel* m, mjData* d, mjtNum* dst, int* rownnz, int* rowadr, int* colind);