refactor(tui): dedupe handlers and formatter test setup

src/tui/tui-submit-test-helpers.ts

@@ -0,0 +1,32 @@
+import { vi } from "vitest";
+import { createEditorSubmitHandler } from "./tui.js";
+
+type MockFn = ReturnType<typeof vi.fn>;
+
+export type SubmitHarness = {
+  editor: {
+    setText: MockFn;
+    addToHistory: MockFn;
+  };
+  handleCommand: MockFn;
+  sendMessage: MockFn;
+  handleBangLine: MockFn;
+  onSubmit: (text: string) => void;
+};
+
+export function createSubmitHarness(): SubmitHarness {
+  const editor = {
+    setText: vi.fn(),
+    addToHistory: vi.fn(),
+  };
+  const handleCommand = vi.fn();
+  const sendMessage = vi.fn();
+  const handleBangLine = vi.fn();
+  const onSubmit = createEditorSubmitHandler({
+    editor,
+    handleCommand,
+    sendMessage,
+    handleBangLine,
+  });
+  return { editor, handleCommand, sendMessage, handleBangLine, onSubmit };
+}