Move meshdir and texturedir to mjsCompiler.

For backwards compatibility in the Python bindings, both `spec.compiler.meshdir` and `spec.meshdir` are now allowed.

Fixes #2834.

PiperOrigin-RevId: 809060113
Change-Id: Ie23f1c5dd57de43568312a8e062906f4013bb474

test/user/user_api_test.cc

@@ -2631,7 +2631,7 @@ TEST_F(MujocoTest, DifferentOptionsInAttachedFrame) {
   mj_deleteModel(m_attached);
 }
 
-TEST_F(MujocoTest, CopyAttachedSpec) {
+TEST_F(MujocoTest, NotCopyAttachedSpec) {
   static constexpr char xml_parent[] = R"(
   <mujoco>
     <asset>
@@ -2675,7 +2675,7 @@ TEST_F(MujocoTest, CopyAttachedSpec) {
 
   mjSpec* child_copy = mjs_findSpec(copy, "child");
   EXPECT_THAT(child_copy, NotNull());
-  EXPECT_NE(child_copy, child);
+  EXPECT_EQ(child_copy, child);
 
   mj_deleteSpec(spec);
   mj_deleteSpec(copy);