Add MjcPhysicsSiteAPI support and testing.

PiperOrigin-RevId: 759124593
Change-Id: Ic2fa2a00f8384f432ee2eef95c80f5c180aa4ca9

src/experimental/usd/plugins/mjcf/mujoco_to_usd.cc

@@ -21,6 +21,7 @@
 #include <vector>
 
 #include <mujoco/mujoco.h>
+#include "third_party/mujoco/src/experimental/usd/mjcPhysics/tokens.h"
 #include "mjcf/utils.h"
 #include <pxr/base/arch/attributes.h>
 #include <pxr/base/gf/matrix4d.h>
@@ -834,6 +835,8 @@ class ModelWriter {
     pxr::SdfPath site_path = WriteSiteGeom(site, body_path);
     SetPrimPurpose(data_, site_path, pxr::UsdGeomTokens->guide);
 
+    ApplyApiSchema(data_, site_path, pxr::MjcPhysicsTokens->SiteAPI);
+
     int site_id = mjs_getId(site->element);
     auto transform = MujocoPosQuatToTransform(&model_->site_pos[3 * site_id],
                                               &model_->site_quat[4 * site_id]);