test: verify reference_internal policy for const-ref returns

Adds test_const_ref_return_policy that explicitly checks:
- Methods returning const T& emit `-> const auto&` trailing return
- Methods returning const T& emit `py::return_value_policy::reference_internal`
- Methods returning by value (T, not T&) do NOT get the policy

Author: jashshah999

tests/test_pybind_wrapper.py

@@ -160,5 +160,41 @@ def test_enum(self):
         self.compare_and_diff('enum_pybind.cpp', output)
 
 
+    def test_const_ref_return_policy(self):
+        """Test that methods returning const T& emit reference_internal policy.
+
+        Without this policy, pybind11 defaults to copying the returned reference.
+        With the policy, the binding keeps the reference alive via the parent object.
+
+        Expected emitted code difference:
+          Before: [](Cls* self, ...){return self->method(...);}, py::arg(...))
+          After:  [](Cls* self, ...) -> const auto&{return self->method(...);},
+                  py::return_value_policy::reference_internal, py::arg(...))
+        """
+        source = osp.join(self.INTERFACE_DIR, 'class.i')
+        output = self.wrap_content([source], 'class_py',
+                                   self.PYTHON_ACTUAL_DIR)
+
+        with open(output, 'r') as f:
+            content = f.read()
+
+        # const Vector& return_vector2 should have reference_internal
+        self.assertIn('-> const auto&{return self->return_vector2', content)
+        self.assertIn('py::return_value_policy::reference_internal', content)
+
+        # const Matrix& return_matrix2 should also have reference_internal
+        self.assertIn('-> const auto&{return self->return_matrix2', content)
+
+        # Non-ref returns (e.g. return_vector1 which returns by value) should NOT
+        lines = content.split('\n')
+        for line in lines:
+            if 'return_vector1' in line:
+                self.assertNotIn('reference_internal', line)
+                self.assertNotIn('-> const auto&', line)
+            if 'return_matrix1' in line:
+                self.assertNotIn('reference_internal', line)
+                self.assertNotIn('-> const auto&', line)
+
+
 if __name__ == '__main__':
     unittest.main()