Set autocomplete="off" on the helper textarea (#6057)

Without an autocomplete hint, focusing a terminal on iOS/iPadOS Safari
pops the AutoFill accessory (passwords / credit cards / contacts) for
the hidden textarea. The sibling hints (autocorrect, autocapitalize,
spellcheck) are already set; this adds the missing one.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/browser/CoreBrowserTerminal.ts

@@ -492,6 +492,7 @@ export class CoreBrowserTerminal extends CoreTerminal implements ITerminal {
       // https://issuetracker.google.com/issues/260170397
       this.textarea.setAttribute('aria-multiline', 'false');
     }
+    this.textarea.setAttribute('autocomplete', 'off');
     this.textarea.setAttribute('autocorrect', 'off');
     this.textarea.setAttribute('autocapitalize', 'off');
     this.textarea.setAttribute('spellcheck', 'false');