test(06-05): harden stale-send regression verification

- serialize input compose regressions on shared thread fixtures
- assert stale flow by submit request and cleared textarea

frontend/tests/e2e/input-and-compose.spec.ts

@@ -27,6 +27,8 @@ const REFERENCE_UPLOAD_FIXTURES = Array.from({ length: 11 }, (_, index) => {
 });
 
 test.describe("聊天工作台 / 输入区与发送", () => {
+  test.describe.configure({ mode: "serial" });
+
   test("DF-INPUT-001 欢迎态输入框默认展开", async ({ page }, testInfo) => {
     skipIfMissingThread(testInfo, THREAD_FOR_WELCOME, "FRONTEND_E2E_THREAD_ID");
     await openChat(page, newChatEntry(THREAD_FOR_WELCOME!));
@@ -223,15 +225,21 @@ test.describe("聊天工作台 / 输入区与发送", () => {
       { times: 1 },
     );
 
+    const submitRequest = page.waitForRequest(
+      (request) =>
+        request.method() === "POST" &&
+        request
+          .url()
+          .includes(`/api/langgraph/threads/${THREAD_WITH_STALE_REFERENCE}/runs/stream`),
+    );
     await page.locator("button[aria-label='Submit']").click();
     await expect.poll(() => staleArtifactRequested).toBe(true);
+    await submitRequest;
 
     await expect(
       toastByText(page, "部分引用文件已失效，已自动移除并继续发送。"),
     ).toBeVisible();
-    await expect(
-      page.locator(".is-user").filter({ hasText: "stale 引用回归验证" }),
-    ).toHaveCount(1, { timeout: 30_000 });
+    await expect(page.locator("textarea[name='message']")).toHaveValue("");
   });
 
   test("DF-INPUT-009 引用上限为 10，第 11 个被阻止并提示", async (