Say that duplicate SubTree names are checked across the whole tree (fixes #1219) (#1222)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: dv-picknik

src/xml_parsing.cpp

@@ -1226,8 +1226,8 @@ void BT::XMLParser::PImpl::recursivelyCreateSubtree(
         if(sub->instance_name == subtree_path)
         {
           throw RuntimeError("Duplicate SubTree path detected: '", subtree_path,
-                             "'. Multiple SubTree nodes with the same 'name' attribute "
-                             "under the same parent are not allowed. "
+                             "'. SubTree nodes in the same tree cannot share a 'name' "
+                             "attribute, even under different parent nodes. "
                              "Please use unique names or omit the 'name' attribute "
                              "to auto-generate unique paths.");
         }

tests/gtest_subtree.cpp

@@ -815,7 +815,7 @@ TEST(SubTree, SubstringTreeIDsAreNotRecursive)
 // Test for Groot2 issue #56: duplicate _fullpath when multiple subtrees have the same name
 // https://github.com/BehaviorTree/Groot2/issues/56
 //
-// When two SubTree nodes under the same parent have the same "name" attribute,
+// When two SubTree nodes in the same tree have the same "name" attribute,
 // tree creation should fail with a clear error message.
 TEST(SubTree, DuplicateSubTreeName_Groot2Issue56)
 {
@@ -879,6 +879,48 @@ TEST(SubTree, DuplicateSubTreeName_ErrorMessage)
   }
 }
 
+// The duplicate check covers the whole tree, so the message must not suggest
+// that moving one SubTree under another parent fixes it.
+TEST(SubTree, DuplicateSubTreeName_DifferentParents)
+{
+  // clang-format off
+  static const char* xml_text = R"(
+<root BTCPP_format="4" main_tree_to_execute="MainTree">
+    <BehaviorTree ID="MainTree">
+        <Fallback>
+            <Sequence>
+                <SubTree ID="Task" name="my_task"/>
+            </Sequence>
+            <Sequence>
+                <SubTree ID="Task" name="my_task"/>
+            </Sequence>
+        </Fallback>
+    </BehaviorTree>
+
+    <BehaviorTree ID="Task">
+        <AlwaysSuccess/>
+    </BehaviorTree>
+</root>
+)";
+  // clang-format on
+
+  BehaviorTreeFactory factory;
+
+  try
+  {
+    (void)factory.createTreeFromText(xml_text);
+    FAIL() << "Expected RuntimeError to be thrown";
+  }
+  catch(const RuntimeError& e)
+  {
+    std::string msg = e.what();
+    EXPECT_TRUE(msg.find("Duplicate SubTree path") != std::string::npos)
+        << "Error message should mention 'Duplicate SubTree path'. Got: " << msg;
+    EXPECT_TRUE(msg.find("same parent") == std::string::npos)
+        << "Error message should not limit the rule to one parent. Got: " << msg;
+  }
+}
+
 // Test that unique names under the same parent work correctly
 TEST(SubTree, UniqueSubTreeNames_WorksCorrectly)
 {