test: add merge edge case tests

Add test classes for list replacement, type promotion, path creation, and no-op merge scenarios. Fix _create_at to correctly handle nested dict values by using placeholder+replace instead of direct Op.add which flattens nested structures.

Author: nathanjmcdougall

tests/test_merge.py

@@ -24,3 +24,53 @@ def test_deeply_nested_merge(self):
         assert doc2["a", "b", "c"] == 99
         assert doc2["a", "b", "d"] == 2
         assert doc2["a", "e"] == 3
+
+
+class TestMergeListReplacement:
+    def test_list_replaced_entirely(self):
+        doc = Document("items:\n  - a\n  - b\n  - c\n")
+        doc2 = doc.merge("items", value=["x"])
+        assert doc2["items"] == ["x"]
+
+    def test_nested_list_replaced(self):
+        doc = Document("cfg:\n  tags:\n    - alpha\n    - beta\n  name: foo\n")
+        doc2 = doc.merge("cfg", value={"tags": ["gamma"]})
+        assert doc2["cfg", "tags"] == ["gamma"]
+        assert doc2["cfg", "name"] == "foo"
+
+
+class TestMergeTypePromotion:
+    def test_scalar_to_mapping(self):
+        doc = Document("settings: defaults\n")
+        doc2 = doc.merge("settings", value={"debug": True})
+        assert doc2["settings", "debug"] is True
+
+    def test_mapping_to_scalar(self):
+        doc = Document("settings:\n  debug: true\n")
+        doc2 = doc.merge("settings", value="off")
+        assert doc2["settings"] == "off"
+
+
+class TestMergePathCreation:
+    def test_creates_missing_path(self):
+        doc = Document("a: 1\n")
+        doc2 = doc.merge("new_section", value={"x": 1})
+        assert doc2["new_section", "x"] == 1
+        assert doc2["a"] == 1
+
+    def test_creates_nested_missing_path(self):
+        doc = Document("a: 1\n")
+        doc2 = doc.merge("b", "c", value={"d": 2})
+        assert doc2["b", "c", "d"] == 2
+
+
+class TestMergeNoop:
+    def test_returns_self_when_equal(self):
+        doc = Document("a: 1\nb: 2\n")
+        doc2 = doc.merge(value={"a": 1, "b": 2})
+        assert doc2 is doc
+
+    def test_returns_self_when_subset(self):
+        doc = Document("a: 1\nb: 2\nc: 3\n")
+        doc2 = doc.merge(value={"a": 1, "b": 2})
+        assert doc2 is doc