Check that there are no pending keyframes while writing a spec.

This can happen when a body is detached without compiling or recompiling.

Also added error catching when trying to delete a body instead of detaching it.

Fixes #2327

PiperOrigin-RevId: 712526730
Change-Id: I4b48df83120fca12d475c85b3a54893841449653

test/user/user_api_test.cc

@@ -1222,9 +1222,21 @@ void TestDetachBody(bool compile) {
   mjsBody* body = mjs_findBody(child, "body");
   EXPECT_THAT(body, NotNull());
 
+  // get an error if trying to delete the body
+  EXPECT_EQ(mjs_delete(body->element), -1);
+  EXPECT_THAT(mjs_getError(child), HasSubstr("use detach instead"));
+
   // detach subtree
   EXPECT_THAT(mjs_detachBody(child, body), 0);
 
+  // try saving to XML before compiling again
+  std::array<char, 1024> e;
+  std::array<char, 1024> s;
+  EXPECT_EQ(mj_saveXMLString(child, s.data(), 1024, e.data(), 1024), -1);
+  EXPECT_THAT(e.data(), compile
+                  ? HasSubstr("Model has pending keyframes")
+                  : HasSubstr("Only compiled model can be written"));
+
   // compile new model
   mjModel* m_detached = mj_compile(child, 0);
   EXPECT_THAT(m_detached, NotNull());