Add support for vfs assets in mj_compile function to MuJoCo WASM.

Regarding mj_compile VFS argument:
I was encountering a memory error when using `std::optional<MjVFS>` with `mj_compile` (because vfs should be an optional argument of the function). This is likely because `MjVFS` doesn't have copy/move constructors, which are required for types used with `std::optional`, causing issues with memory management when std::optional copies the MjVFS object.
To solve this and to make vfs an optional argument to `mj_compile`, I replaced `mj_compile_wrapper` with two overloaded functions. This avoids using `std::optional<MjVFS>` and resolves the memory issue, while still allowing mj_compile to be called with or without the vfs argument from JavaScript.

But what if we instead add those copy/move constructors so it works with `std::optional`?:
While making `MjVFS` copyable or movable would theoretically allow it to work with std::optional, it's not a feasible approach in this case.
The MjVFS struct manages file data in memory, allocating buffers when files are added and freeing them when `mj_deleteVFS` is called in its destructor. A copy would require a deep copy of the entire virtual filesystem, but MuJoCo does not provide a public API function (like a  "mj_copyVFS") to do this. Implementing a deep copy manually would be complex and fragile. A shallow copy would result in multiple MjVFS objects pointing to the same memory, leading to double-free errors when their destructors run.
Given these constraints, MjVFS cannot safely be made copyable. The approach of providing two overloaded C++ functions is better.

PiperOrigin-RevId: 855740541
Change-Id: Ia08b6db8868ad245f07145db3fb0322cb9f737cb

wasm/tests/bindings_test.ts

@@ -17,7 +17,7 @@ import 'jasmine';
 import {MainModule, MjContact, MjContactVec, MjData, MjLROpt, MjModel,
 MjOption, MjsGeom, MjSolverStat, MjSpec, MjStatistic, MjTimerStat, MjvCamera,
 MjvFigure, MjvGeom, MjvGLCamera, MjvLight, MjvOption, MjvPerturb, MjvScene,
-MjWarningStat} from '../dist/mujoco_wasm.js';
+MjWarningStat, MjVFS} from '../dist/mujoco_wasm.js';
 
 import loadMujoco from '../dist/mujoco_wasm.js'
 
@@ -1803,7 +1803,7 @@ describe('MuJoCo WASM Bindings', () => {
     }
   });
 
-  it('should compile a spec from XML string', () => {
+  it('should compile a spec from XML string with no assets', () => {
     let spec = null;
     let model = null;
     try {
@@ -1826,4 +1826,50 @@ describe('MuJoCo WASM Bindings', () => {
       }
     }
   });
+
+  it('should compile a spec from XML with .obj asset', () => {
+    const xml = `
+    <mujoco>
+      <asset>
+        <mesh file="cube.obj"/>
+      </asset>
+      <worldbody>
+        <geom type="mesh" mesh="cube"/>
+      </worldbody>
+    </mujoco>`;
+
+    const cube1 = `
+    v -1 -1  1
+    v  1 -1  1
+    v -1  1  1
+    v  1  1  1
+    v -1  1 -1
+    v  1  1 -1
+    v -1 -1 -1
+    v  1 -1 -1`;
+
+    let spec: MjSpec|null = null;
+    let model: MjModel|null = null;
+    let vfs: MjVFS|null = null;
+    try {
+      spec = mujoco.parseXMLString(xml);
+      assertExists(spec);
+
+      vfs = new mujoco.MjVFS();
+      vfs.addBuffer('cube.obj', new TextEncoder().encode(cube1));
+      assertExists(vfs);
+
+      model = mujoco.mj_compile(spec, vfs);
+      assertExists(model);
+      expect(model.nmesh).toBe(1);
+
+      const meshId =
+          mujoco.mj_name2id(model, mujoco.mjtObj.mjOBJ_MESH.value, 'cube');
+      expect(meshId).toBeGreaterThanOrEqual(0);
+    } finally {
+      spec?.delete();
+      vfs?.delete();
+      model?.delete();
+    }
+  });
 });