Engine: Save Model asks before it writes over a Python cell

Save Model (new on main) asks before it replaces data in the column it writes
to, but it looked for a value or a formula only, so a Python cell whose cached
value is empty was taken for an empty cell and its code was replaced without
the question. Python code now counts as content there, as it does elsewhere.

crates/engine/src/tests/python_cells.rs

@@ -349,3 +349,25 @@ fn unmerging_lets_a_blocked_python_result_spill_again() {
     s.execute("home.unmergeCells", json!({"range": "A2:B2"})).unwrap();
     assert_eq!((super::v(&s, "A1"), super::v(&s, "A2")), (Value::Number(5.0), Value::Number(6.0)), "it spills again");
 }
+
+#[test]
+fn save_model_asks_before_it_writes_over_a_python_cell_that_shows_nothing() {
+    let mut s = s();
+    for (cell, input) in [("B1", "=5*A1+4*A2"), ("B2", "=6*A1+4*A2")] {
+        s.execute("cell.set", json!({"cell": cell, "input": input})).unwrap();
+    }
+    // H2 is a Python cell whose cached value is empty: its code is still content.
+    {
+        let d = s.doc_mut().unwrap();
+        let wb = Arc::make_mut(&mut d.wb);
+        let sh = wb.sheet_mut(0).unwrap();
+        sh.set_cell(
+            CellRef::parse("H2").unwrap(),
+            Cell { value: Value::Empty, formula: None, style: Default::default(), py: Some(Arc::new(PyCode::new("None"))) },
+        );
+    }
+    let model = json!({"objective": "B1", "variables": "A1:A2", "constraints": ["B2 <= 24"]});
+    let e = s.execute("data.solverSaveModel", json!({"to": "H1", "model": model})).unwrap_err().to_string();
+    assert!(e.contains(crate::cmd::analysis::OVERWRITE_ERROR) && e.contains("H2"), "{e}");
+    assert_eq!(code(&s, 0, "H2").as_deref(), Some("None"), "nothing was written");
+}