fix(sp25): typecheck fixups

Three small follow-ups after the live-tail triplet wired up:

  * evictOldest is now generic over (K extends string|number, V) so
    the addAck / addTa1Ack call sites don't need a type assertion.
  * Acks.test.tsx no longer spreads importOriginal() — TypeScript
    was choking on the inferred 'unknown' from the mock factory.
    Use the same ApiError / mock shape that Claims.test.tsx uses.
  * tail-stream.test.ts guards the optional gen.return() with
    so strict mode TS2722 stops complaining about possibly-undefined.

src/pages/Acks.test.tsx

@@ -9,15 +9,19 @@ import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import { Acks } from "./Acks";
 import { api } from "@/lib/api";
 
-vi.mock("@/lib/api", async (importOriginal) => {
-  const actual = await importOriginal();
+vi.mock("@/lib/api", () => {
+  class ApiError extends Error {
+    constructor(public status: number, message: string) {
+      super(message);
+    }
+  }
   return {
-    ...actual,
     api: {
       isConfigured: true,
       listAcks: vi.fn(),
       getAck: vi.fn(),
     },
+    ApiError,
   };
 });