Add undo/redo support to editable CodeBlock (#3288)

* Add undo/redo support to editable CodeBlock

Enable CodeMirror's history() extension so existing historyKeymap
keybindings (Cmd+Z / Cmd+Shift+Z) actually work. Annotate
programmatic content replacements with addToHistory: false to keep
prop syncs and blur reformats out of the undo stack.

* Load history() in dynamicExtensions only if code block is editable

* Remove duplicate onblur

---------

Co-authored-by: Laura Whitaker <laura.whitaker@temporal.io>

Author: Alex-Tideman

src/lib/holocene/code-block.svelte

@@ -2,7 +2,7 @@
   import type { HTMLAttributes } from 'svelte/elements';
 
   import { autocompletion, closeBrackets } from '@codemirror/autocomplete';
-  import { historyKeymap, standardKeymap } from '@codemirror/commands';
+  import { history, historyKeymap, standardKeymap } from '@codemirror/commands';
   import {
     bracketMatching,
     ensureSyntaxTree,
@@ -11,7 +11,12 @@
     indentUnit,
     syntaxHighlighting,
   } from '@codemirror/language';
-  import { Compartment, EditorState, type Extension } from '@codemirror/state';
+  import {
+    Compartment,
+    EditorState,
+    type Extension,
+    Transaction,
+  } from '@codemirror/state';
   import { EditorView, keymap } from '@codemirror/view';
   import { onMount, type Snippet } from 'svelte';
   import { twMerge as merge, twMerge } from 'tailwind-merge';
@@ -116,6 +121,7 @@
           to: doc.length,
           insert: newContent,
         },
+        annotations: Transaction.addToHistory.of(false),
       });
     }
   };
@@ -156,6 +162,7 @@
 
   let dynamicExtensions: Extension[] = $derived(
     [
+      ...(editable ? [history()] : []),
       getEditorTheme($useDarkMode, hasHeader),
       getActionsTheme({ hasActions: copyable || maximizable }),
       EditorState.readOnly.of(!editable),
@@ -285,7 +292,6 @@
       class={merge('h-full', className)}
       data-testid={testId}
       {...editorProps}
-      onblur={handleEditorBlur}
     ></div>
 
     {#snippet actions()}