documents: one undo history; a block's own never runs

Undo or redo in an open block writes it back as one step and steps the
document, so redo survives mode switches and further undos.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/documents/document/edit.rs

@@ -1,6 +1,6 @@
 use std::ops::Range;
 
-use gpui::{App, AppContext as _, Context, Focusable as _, Window};
+use gpui::{AppContext as _, Context, Focusable as _, Window};
 
 use super::{DocumentEvent, MarkdownDocument, Open};
 use crate::{
@@ -31,13 +31,6 @@ impl MarkdownDocument {
         }
     }
 
-    /// The open block has no step left that way.
-    pub(super) fn at_edge_of_history(&self, redo: bool, cx: &App) -> bool {
-        self.open
-            .as_ref()
-            .is_some_and(|open| !open.input.read(cx).can_step(redo))
-    }
-
     /// Undo or redo in the document's one history.
     pub(super) fn history(&mut self, redo: bool, window: &mut Window, cx: &mut Context<Self>) {
         self.commit(cx);