test(examples-chat): aimock regenerate scenario (#336)

Sends a prompt, waits for the assistant turn to finalize, clicks
Regenerate response, asserts conversation count stays at 1 user / 1
assistant after the regenerated turn finalizes. Reuses the existing
'say hi briefly' fixture — aimock returns the same response on the
regenerated call and the in-place replacement invariant is what we
assert against.

Author: blove

examples/chat/aimock-e2e/regenerate.spec.ts

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: MIT
+import { test, expect } from '@playwright/test';
+import { sendPromptAndWait } from './test-helpers';
+
+test('regenerate: re-running keeps 1 user / 1 assistant in the conversation', async ({
+  page,
+}) => {
+  // Reuse the smoke 'say hi briefly' fixture — aimock returns the same
+  // response on regenerate; the invariant we care about is the count.
+  await sendPromptAndWait(page, 'say hi briefly');
+
+  const userMessages = page.locator('chat-message[data-role="user"]');
+  const assistantMessages = page.locator('chat-message[data-role="assistant"]');
+  await expect(userMessages).toHaveCount(1);
+  await expect(assistantMessages).toHaveCount(1);
+
+  // Click Regenerate on the assistant message (aria-label is the durable hook).
+  await page.getByRole('button', { name: 'Regenerate response' }).first().click();
+
+  // Wait for the regenerated assistant turn to finalize (data-streaming flips
+  // back to true then false). We can't reuse sendPromptAndWait here because
+  // there's no fresh prompt to send — instead poll until exactly one
+  // finalized assistant is present and the count holds at 1/1.
+  await expect
+    .poll(
+      async () =>
+        page
+          .locator('chat-message[data-role="assistant"][data-streaming="false"]')
+          .count(),
+      { timeout: 45_000 },
+    )
+    .toBeGreaterThan(0);
+
+  // Single-turn invariant: after regenerate, conversation stays at 1u/1a
+  // (the assistant message is replaced in place, not appended).
+  await expect(userMessages).toHaveCount(1);
+  await expect(assistantMessages).toHaveCount(1);
+});