Add mjmBody and mjmPlugin and their compiler C APIs.

PiperOrigin-RevId: 604937231
Change-Id: Ib8a2105ce6c0500d6a610258b46f192d9e73169d

test/engine/engine_plugin_test.cc

@@ -17,6 +17,7 @@
 
 #include "src/engine/engine_plugin.h"
 
+#include <array>
 #include <cstdint>
 #include <sstream>
 #include <string>
@@ -421,6 +422,22 @@ constexpr char xml[] = R"(
 </mujoco>
 )";
 
+TEST_F(MujocoTest, EmptyPluginDisallowed) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <worldbody>
+      <plugin/>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* m = LoadModelFromString(xml, error.data(), error.size());
+  EXPECT_THAT(m, testing::IsNull()) << error.data();
+  EXPECT_THAT(error.data(), HasSubstr(
+              "neither 'plugin' nor 'instance' is specified for body 'world'"));
+  mj_deleteModel(m);
+}
+
 TEST_F(PluginTest, FirstPartyPlugins) {
   EXPECT_THAT(mjp_pluginCount(), kNumTruePlugins);
 }

test/user/user_api_test.cc

@@ -18,7 +18,6 @@
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 #include "src/user/user_api.h"
-#include <mujoco/mjmodel.h>
 #include "test/fixture.h"
 
 namespace mujoco {
@@ -31,8 +30,8 @@ using ::testing::HasSubstr;
 
 TEST_F(MujocoTest, ReadWriteData) {
   void* model = mjm_createModel();
-  void* world = mjm_findObject(model, mjOBJ_BODY, "world");
-  void* body = mjm_addBody(world, 0);
+  mjmBody* world = mjm_findBody(model, "world");
+  mjmBody* body = mjm_addBody(world, 0);
   mjmSite* site = mjm_addSite(body, 0);
 
   {