test(app/e2e): stream auto-scroll and working-indicator→copy-button (Cluster G5) (#743)

* test(app/e2e): stream auto-scroll and working-indicator→copy-button (Cluster G5)

Wire in the unused agent-bottom-anchor helpers and add two new agent stream
UI specs: auto-scroll stays pinned to the bottom across token bursts, and
the inline working-indicator transitions to a copy-button when the stream
ends. Both tests use the mock provider so there is no real LLM dependency
in CI.

Adds three helpers to helpers/agent-stream.ts:
- expectInlineWorkingIndicator
- expectTurnCopyButton
- expectScrolledToBottom (wraps agent-bottom-anchor)

* fixup: address review blockers and nits

- Replace expectScrolledToBottom passthrough with expectScrollFollowsNewContent
  that encapsulates readScrollMetrics → waitForContentGrowth → expectNearBottom
- Remove direct agent-bottom-anchor imports from spec body (DSL leak)
- Add awaitAssistantMessage before expectInlineWorkingIndicator to anchor on
  real content before asserting the working indicator
- Add comment to expectInlineWorkingIndicator explaining why testId is used
  (animated spinner View has no ARIA role)

packages/app/e2e/agent-stream-ui.spec.ts

@@ -0,0 +1,45 @@
+import { test } from "./fixtures";
+import {
+  awaitAssistantMessage,
+  expectAgentIdle,
+  expectInlineWorkingIndicator,
+  expectTurnCopyButton,
+  expectScrollFollowsNewContent,
+} from "./helpers/agent-stream";
+import { startRunningMockAgent } from "./helpers/composer";
+
+test.describe("Agent stream UI", () => {
+  test("auto-scroll sticks to bottom across token bursts", async ({ page }) => {
+    test.setTimeout(120_000);
+    const { client, repo } = await startRunningMockAgent(page, {
+      prefix: "stream-scroll-",
+      model: "one-minute-stream",
+      prompt: "Stream for auto-scroll test.",
+    });
+    try {
+      await awaitAssistantMessage(page);
+      await expectScrollFollowsNewContent(page);
+    } finally {
+      await client.close();
+      await repo.cleanup();
+    }
+  });
+
+  test("working-indicator transitions to copy-button when stream ends", async ({ page }) => {
+    test.setTimeout(60_000);
+    const { client, repo } = await startRunningMockAgent(page, {
+      prefix: "stream-indicator-",
+      model: "ten-second-stream",
+      prompt: "Stream briefly for indicator transition test.",
+    });
+    try {
+      await awaitAssistantMessage(page);
+      await expectInlineWorkingIndicator(page);
+      await expectAgentIdle(page, 30_000);
+      await expectTurnCopyButton(page);
+    } finally {
+      await client.close();
+      await repo.cleanup();
+    }
+  });
+});