Migrate mjModel size fields from int to mjtSize.

This change updates all size-related members within the `mjModel` struct from `int` to `mjtSize`. This allows MuJoCo to handle models with a larger number of elements. Corresponding changes were made to macros, function signatures, and I/O routines to accommodate the new `mjtSize` type.

PiperOrigin-RevId: 860144595
Change-Id: I701c6d607715d240766b6210a9773cd9e4258c59

python/mujoco/structs.cc

@@ -56,9 +56,13 @@ namespace {
 // (dim0, dim1).
 #define X_ARRAY_SHAPE(dim0, dim1) XArrayShapeImpl(#dim1)((dim0), (dim1))
 
-std::vector<int> XArrayShapeImpl1D(int dim0, int dim1) { return {dim0}; }
+std::vector<mjtSize> XArrayShapeImpl1D(mjtSize dim0, mjtSize dim1) {
+  return {dim0};
+}
 
-std::vector<int> XArrayShapeImpl2D(int dim0, int dim1) { return {dim0, dim1}; }
+std::vector<mjtSize> XArrayShapeImpl2D(mjtSize dim0, mjtSize dim1) {
+  return {dim0, dim1};
+}
 
 constexpr auto XArrayShapeImpl(const std::string_view dim1_str) {
   if (dim1_str == "1") {
@@ -401,20 +405,20 @@ This is useful for example when the MJB is not available as a file on disk.)"));
 #define X(var)                   \
   mjModel.def_property_readonly( \
       #var, [](const MjModelWrapper& m) { return m.get()->var; });
-  MJMODEL_INTS
+  MJMODEL_SIZES
 #undef X
 
   mjModel.def_property_readonly("_sizes", [](const MjModelWrapper& m) {
     int nint = 0;
 #define X(var) ++nint;
-    MJMODEL_INTS
+    MJMODEL_SIZES
 #undef X
     py::array_t<std::int64_t> sizes(nint);
     {
       int i = 0;
       auto data = sizes.mutable_unchecked();
 #define X(var) data[i++] = m.get()->var;
-      MJMODEL_INTS
+      MJMODEL_SIZES
 #undef X
     }
     py::detail::array_proxy(sizes.ptr())->flags &=
@@ -425,7 +429,7 @@ This is useful for example when the MJB is not available as a file on disk.)"));
   mjModel.def_property_readonly_static("_size_fields", [](py::object) {
     std::vector<std::string> fields;
 #define X(var) fields.push_back(#var);
-    MJMODEL_INTS
+    MJMODEL_SIZES
 #undef X
     return py::tuple(py::cast(fields));
   });
@@ -433,7 +437,7 @@ This is useful for example when the MJB is not available as a file on disk.)"));
   mjModel.def_property_readonly_static("_all_fields", [](py::object) {
     std::vector<std::string> fields;
 #define X(var) fields.push_back(#var);
-    MJMODEL_INTS
+    MJMODEL_SIZES
 #undef X
 #define X(type, name, nr, nc) fields.push_back(#name);
     MJMODEL_POINTERS