Engine: Python code counts as content where a cell's content is checked

A Python cell whose cached value is empty (a file's PY formula with no cached
value) was taken for an empty cell by six checks, which use the value and the
formula only:
- the Analysis ToolPak report and the PivotTable report replaced it without
  asking for overwrite or replace (CodeRabbit, on the report);
- Insert Cells and Insert Table without headers pushed it off the end of the
  sheet;
- the data region (Ctrl+A, AutoSum, sort) ended at it;
- pasting it into part of a legacy array was not refused.
They now use Cell::has_content, which counts Python code.

Test: python_code_with_an_empty_value_counts_as_content; reverting any one of
the six changes makes it fail.

crates/engine/src/tests/python_cells.rs

@@ -484,3 +484,49 @@ fn an_edit_that_would_make_python_code_too_long_is_refused() {
     assert_eq!(code(&s, 0, "A5").unwrap(), a5);
     assert_eq!(code(&s, 1, "C1"), None);
 }
+
+/// Makes `cell` a Python cell whose cached value is empty, as a file's `PY` formula with no cached
+/// value reads.
+fn plant_empty(s: &mut Session, cell: &str) {
+    plant(s, 0, cell, "x = 1");
+    let d = s.doc_mut().unwrap();
+    let mut wb = (*d.wb).clone();
+    if let Some(c) = wb.sheet_mut(0).unwrap().cells.get_mut(CellRef::parse(cell).unwrap()) {
+        c.value = Value::Empty;
+    }
+    d.wb = Arc::new(wb);
+}
+
+#[test]
+fn python_code_with_an_empty_value_counts_as_content() {
+    // A report or a PivotTable asks before replacing it.
+    let mut s = s();
+    s.execute("range.setValues", json!({"range": "A1", "values": [["Region", "Sales"], ["East", 1], ["West", 2], ["East", 3]]})).unwrap();
+    plant_empty(&mut s, "D2");
+    let report = json!({"tool": "Descriptive Statistics", "range": "B1:B4", "labels": true, "output": "D1"});
+    assert!(s.execute("data.analysisToolPak", report).is_err(), "the report asks for overwrite");
+    plant_empty(&mut s, "H4");
+    let pivot = s.execute("insert.pivotTable", json!({"destination": "Sheet1!H3", "rows": ["Region"], "values": ["Sales"]}));
+    assert!(pivot.is_err_and(|e| e.to_string().contains("already data")));
+    assert_eq!((code(&s, 0, "D2"), code(&s, 0, "H4")), (Some("x = 1".into()), Some("x = 1".into())), "the code is kept");
+    // Cells aren't pushed off the sheet over it.
+    plant_empty(&mut s, "K1048576");
+    assert!(s.execute("home.insertCells", json!({"range": "K1", "shift": "down"})).is_err());
+    assert_eq!(code(&s, 0, "K1048576"), Some("x = 1".into()));
+    // A table without a header row doesn't push it off the sheet either.
+    s.execute("range.setValues", json!({"range": "X1", "values": [[1], [2]]})).unwrap();
+    plant_empty(&mut s, "X1048576");
+    assert!(s.execute("insert.table", json!({"range": "X1:X2", "header": false})).is_err());
+    assert_eq!(code(&s, 0, "X1048576"), Some("x = 1".into()));
+    // It is part of the data region.
+    s.execute("range.setValues", json!({"range": "P1", "values": [[1], [""], [3]]})).unwrap();
+    plant_empty(&mut s, "P2");
+    let sh = s.doc().unwrap().wb.sheet(0).unwrap().clone();
+    assert_eq!(crate::selection::current_region(&sh, CellRef::parse("P1").unwrap()).a1(), "P1:P3");
+    // Pasted into part of a legacy array, it is refused like any content.
+    s.execute("selection.set", json!({"range": "R1:R3"})).unwrap();
+    s.execute("cell.set", json!({"input": "=5", "array": true})).unwrap();
+    plant_empty(&mut s, "T1");
+    s.execute("edit.copy", json!({"range": "T1"})).unwrap();
+    assert!(s.execute("edit.paste", json!({"at": "R2"})).is_err_and(|e| e.to_string().contains("part of an array")));
+}