test: insert newlines as text instead of pressing Enter

Monaco looks up word-based quick suggestions asynchronously in the
editor worker. At synthetic typing speed the list can still show the
word being typed when Enter arrives, and Enter then accepts it as a
no-op replacement instead of breaking the line. That dropped the
newline in "should update diagram after entire text is removed" about
one run in eight once keystrokes stopped rendering synchronously.
keyboard.insertText dispatches only an input event, so no suggestion
can be accepted.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/test.ts

@@ -30,7 +30,17 @@ export class EditorPage {
     if (newline) {
       await this.page.keyboard.press('Enter');
     }
-    await this.page.keyboard.type(text, { delay: 10 });
+    // Line breaks are inserted as text rather than pressed as Enter: Monaco's
+    // word-based quick suggestions resolve asynchronously, and an Enter that
+    // lands while the list is open accepts the suggestion instead of breaking
+    // the line, which at synthetic typing speed happens often enough to flake.
+    const lines = text.split('\n');
+    for (const [index, line] of lines.entries()) {
+      if (index > 0) {
+        await this.page.keyboard.insertText('\n');
+      }
+      await this.page.keyboard.type(line, { delay: 10 });
+    }
   }
 
   async clearEditor() {