test: Add test for chained extends with undefined parent path

Verifies that when a grandchild template extends a child template
(resolved path), and the child extends a parent (undefined path),
the child's extends and block tags are correctly preserved.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: hs-lsong

src/test/java/com/hubspot/jinjava/mode/PreserveUndefinedExecutionModeTest.java

@@ -280,6 +280,26 @@ public void itPreservesUndefinedVariablesInExtendedTemplate() {
     assertThat(output).isEqualTo("Title: {{ title }} - {{ message }}");
   }
 
+  @Test
+  public void itPreservesNestedExtendsWhenParentPathIsUndefined() {
+    // Chained inheritance: grandchild extends child (resolved), child extends parent (undefined)
+    jinjava.setResourceLocator((fullName, encoding, interpreter) -> {
+      if (fullName.equals("child.html")) {
+        return "{% extends parentPath %}{% block content %}child: {{ childVar }}{% endblock %}";
+      }
+      return "";
+    });
+
+    String template =
+      "{% extends 'child.html' %}{% block content %}grandchild content{% endblock %}";
+    String output = render(template);
+    // The child template's extends and block should be preserved since parentPath is undefined
+    assertThat(output)
+      .isEqualTo(
+        "{% extends parentPath %}{% block content %}child: {{ childVar }}{% endblock %}"
+      );
+  }
+
   @Test
   public void itPreservesComments() {
     String output = render("{# this is a comment #}");