Commands: a boolean or number given as text is refused

Squashed from #3428 onto main.

crates/engine/src/cmd/gradient.rs

@@ -381,6 +381,7 @@ fn edit_gradient(s: &mut Session, p: &Value) -> Result<Value> {
         Some(_) if !stroke => return Err(bad(C, "`strokeMode` applies to strokes (stroke: true)")),
         Some(m) => Some(StrokeGradientMode::parse(m).ok_or_else(|| bad(C, format!("strokeMode must be within|along|across, got {m}")))?),
     };
+    super::paint::check_keep_model(p, C)?;
     // New RGB stop colors take the document's model; those of the swatches the stops name don't.
     let stored = stop_colors(s, p, p);
     // Stops naming swatches take their colours from them.