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

test/header_test.cc

@@ -126,7 +126,7 @@ TEST_F(HeaderTest, MjModelIntsOrdered) {
   std::vector<std::pair<const void*, const char*>> ints;
 
 #define X(name) ints.push_back({static_cast<const void*>(&m.name), #name});
-  MJMODEL_INTS
+  MJMODEL_SIZES
 #undef X
 
   CheckAddressOrdering(ints, "MJMODEL_INT");