make _format_plan respect format_on_save; format enabled/disabled unit tests (#9380)

handles #9348.

---------

Co-authored-by: Myles Scolnick <myles@marimo.io>

Author: ouatu-ro

marimo/_code_mode/_context.py

@@ -1609,7 +1609,10 @@ async def _apply_ops(
     # ------------------------------------------------------------------
 
     async def _format_plan(self, plan: list[_PlanEntry]) -> list[_PlanEntry]:
-        """Format new/changed code in the plan with the default formatter."""
+        """Format new/changed code when save-time formatting is enabled."""
+        if not self._kernel.user_config["save"]["format_on_save"]:
+            return plan
+
         existing_code = {cell.id: cell.code for cell in self._document.cells}
 
         to_format: dict[CellId_t, str] = {}

tests/_code_mode/test_context.py

@@ -327,6 +327,42 @@ async def test_update_config_only(self, k: Kernel) -> None:
                 ]
             )
 
+    async def test_update_code_skips_formatting_when_disabled(
+        self, k: Kernel
+    ) -> None:
+        await k.run([ExecuteCellCommand(cell_id=CellId_t("0"), code="x = 1")])
+
+        with (
+            patch.dict(k.user_config["save"], {"format_on_save": False}),
+            patch(
+                "marimo._code_mode._context.DefaultFormatter.format",
+                new_callable=AsyncMock,
+            ) as mock_format,
+        ):
+            with _ctx(k) as ctx:
+                async with ctx as nb:
+                    nb.edit_cell("0", code="x=  42")
+
+        mock_format.assert_not_awaited()
+        assert _graph_codes(k) == {"0": "x=  42"}
+
+    async def test_update_code_formats_when_enabled(self, k: Kernel) -> None:
+        await k.run([ExecuteCellCommand(cell_id=CellId_t("0"), code="x = 1")])
+
+        with (
+            patch.dict(k.user_config["save"], {"format_on_save": True}),
+            patch(
+                "marimo._code_mode._context.DefaultFormatter.format",
+                new=AsyncMock(return_value={"0": "x = 42"}),
+            ) as mock_format,
+        ):
+            with _ctx(k) as ctx:
+                async with ctx as nb:
+                    nb.edit_cell("0", code="x=  42")
+
+        mock_format.assert_awaited_once()
+        assert _graph_codes(k) == {"0": "x = 42"}
+
 
 class TestCombined:
     async def test_delete_and_add(self, k: Kernel) -> None: