fix: enhance merge logic to handle undefined target values and add corresponding unit tests

Author: techouse

src/qs_codec/utils/utils.py

@@ -328,10 +328,16 @@ def merge(
                 has_source = idx in frame.list_source
 
                 if has_target and has_source:
+                    target_value = frame.list_target[idx]
+                    source_value = frame.list_source[idx]
+
+                    if isinstance(source_value, Undefined):
+                        if not isinstance(target_value, Undefined):
+                            frame.list_merged.append(target_value)
+                        continue
+
                     frame.phase = "list_wait_child"
-                    stack.append(
-                        _MergeFrame(target=frame.list_target[idx], source=frame.list_source[idx], options=frame.options)
-                    )
+                    stack.append(_MergeFrame(target=target_value, source=source_value, options=frame.options))
                     continue
 
                 if has_target:

tests/unit/utils_test.py

@@ -594,6 +594,24 @@ def test_merges_array_into_object(self) -> None:
             {"foo": ["xyzzy"]},
         ) == {"foo": {"bar": "baz", "0": "xyzzy"}}
 
+    def test_merge_mapping_target_with_scalar_source_returns_target_unchanged(self) -> None:
+        target = {"a": "b"}
+        source = "scalar"
+
+        result = Utils.merge(target, source)  # type: ignore[arg-type]
+
+        assert result == {"a": "b"}
+        assert result is target
+
+    def test_merge_structured_lists_prefers_source_when_target_slot_is_undefined(self) -> None:
+        options = DecodeOptions()
+        target = [Undefined()]
+        source = [{"from_source": 1}]
+
+        result = Utils.merge(target, source, options)
+
+        assert result == [{"from_source": 1}]
+
     def test_merge_deep_maps_without_stack_overflow(self) -> None:
         # Keep this above common recursion limits so recursion regressions still fail quickly.
         depth = 12_000