fix: defensive-copy _get_row_types row_types branch

`_get_row_types` returned `self.row_types[row_idx]` by reference on
the per-row-types branch — the same aliasing pattern that issue 052
fixed for the `column_types` fallback. A caller who captured the
return value and mutated it would silently rewrite the message's
internal `row_types[i]` list. Return a fresh `list(...)` copy,
completing the invariant that all three branches of `_get_row_types`
now return independent lists.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqlitewire/messages/responses.py

@@ -254,7 +254,7 @@ def _get_row_types(self, row_idx: int, row: list[Any]) -> list[ValueType]:
         ``__post_init__`` establishes (issue 042, issue 052).
         """
         if self.row_types and row_idx < len(self.row_types):
-            return self.row_types[row_idx]
+            return list(self.row_types[row_idx])
         if self.column_types:
             return list(self.column_types)
         # Infer from values

tests/test_messages_responses.py

@@ -267,6 +267,35 @@ def test_get_row_types_does_not_alias_column_types(self) -> None:
             "msg.column_types (issue 042 invariant)"
         )
 
+    def test_get_row_types_does_not_alias_row_types(self) -> None:
+        """Regression for issue 059.
+
+        The ``row_types`` branch of ``_get_row_types`` used to return
+        ``self.row_types[row_idx]`` by reference — the same aliasing
+        pattern that issue 052 fixed for the ``column_types`` fallback.
+        A caller who captured the return value and mutated it would
+        silently rewrite the message's internal ``row_types[i]`` list.
+        """
+        msg = RowsResponse(
+            column_names=["x"],
+            column_types=[ValueType.INTEGER],
+            row_types=[[ValueType.INTEGER]],
+            rows=[[1]],
+            has_more=False,
+        )
+
+        row_types = msg._get_row_types(0, [1])
+
+        assert row_types is not msg.row_types[0], (
+            "_get_row_types must return a fresh list, not alias row_types[i]"
+        )
+
+        row_types.append(ValueType.BLOB)
+        assert msg.row_types[0] == [ValueType.INTEGER], (
+            "mutating the _get_row_types return value must not affect "
+            "msg.row_types[i] (issue 042 invariant)"
+        )
+
 
 class TestRowsResponse:
     def test_empty_result(self) -> None: