feat: write the library version as BTCPP_version in the <root> of generated XML (#1264)

writeTreeNodesModelXML() and WriteTreeToXML() now add BTCPP_version="<major>.<minor>"
to <root>, next to BTCPP_format. It tells tools like Groot2 which builtin node
models and attributes (e.g. the _onlyIf/_onStart preconditions of 4.11) are
available. The XSD declares it as an optional attribute.

The parser ignores it, as all BT.CPP 4.x releases already ignore unknown <root>
attributes, so files written by 4.11 still load in older versions.

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

Author: facontidavide

src/xml_parsing.cpp

@@ -1454,6 +1454,14 @@ void addTreeToXML(const Tree& tree, XMLDocument& doc, XMLElement* rootXML,
     addNodeModelToXML(*model, doc, model_root);
   }
 }
+
+// "major.minor" of this library, written in the [BTCPP_version] attribute of <root>.
+// It is informative only (e.g. for Groot2): the parser ignores it.
+std::string libraryMajorMinorVersion()
+{
+  const std::string version = LibraryVersionString();
+  return version.substr(0, version.rfind('.'));
+}
 }  // namespace
 
 std::string writeTreeNodesModelXML(const BehaviorTreeFactory& factory,
@@ -1463,6 +1471,7 @@ std::string writeTreeNodesModelXML(const BehaviorTreeFactory& factory,
 
   XMLElement* rootXML = doc.NewElement("root");
   rootXML->SetAttribute("BTCPP_format", "4");
+  rootXML->SetAttribute("BTCPP_version", libraryMajorMinorVersion().c_str());
   doc.InsertFirstChild(rootXML);
 
   XMLElement* model_root = doc.NewElement("TreeNodesModel");
@@ -1641,6 +1650,7 @@ std::string writeTreeXSD(const BehaviorTreeFactory& factory)
         </xs:sequence>
         <xs:attribute name="BTCPP_format" type="xs:string" use="required"/>
         <xs:attribute name="main_tree_to_execute" type="xs:string" use="optional"/>
+        <xs:attribute name="BTCPP_version" type="xs:string" use="optional"/>
       </xs:complexType>
     </xs:element>
   )";
@@ -1765,6 +1775,7 @@ std::string WriteTreeToXML(const Tree& tree, bool add_metadata, bool add_builtin
 
   XMLElement* rootXML = doc.NewElement("root");
   rootXML->SetAttribute("BTCPP_format", 4);
+  rootXML->SetAttribute("BTCPP_version", libraryMajorMinorVersion().c_str());
   doc.InsertFirstChild(rootXML);
 
   addTreeToXML(tree, doc, rootXML, add_metadata, add_builtin_models);

tests/gtest_factory.cpp

@@ -481,6 +481,30 @@ TEST(BehaviorTreeFactory, ManifestMethod)
   EXPECT_NE(xml.find(expectedXML), std::string::npos);
 }
 
+TEST(BehaviorTreeFactory, WrittenXMLHasLibraryVersion)
+{
+  // "4.11.2" -> "4.11"
+  const std::string version = LibraryVersionString();
+  const std::string major_minor = version.substr(0, version.rfind('.'));
+  const std::string expected = "BTCPP_version=\"" + major_minor + "\"";
+
+  BehaviorTreeFactory factory;
+  EXPECT_NE(writeTreeNodesModelXML(factory).find(expected), std::string::npos);
+
+  auto tree = factory.createTreeFromText(R"(
+    <root BTCPP_format="4">
+      <BehaviorTree ID="MainTree">
+        <AlwaysSuccess/>
+      </BehaviorTree>
+    </root>)");
+  const auto xml = WriteTreeToXML(tree, false, false);
+  EXPECT_NE(xml.find(expected), std::string::npos);
+
+  // the attribute is ignored by the parser
+  auto reloaded = factory.createTreeFromText(xml);
+  EXPECT_EQ(reloaded.tickWhileRunning(), NodeStatus::SUCCESS);
+}
+
 TEST(BehaviorTreeFactory, ReturnByValue)
 {
   // Issue #937: BehaviorTreeFactory cannot be returned by value from