fix: make fast-tool stub type portable

src/agents/test-helpers/fast-tool-stubs.ts

@@ -1,10 +1,18 @@
 import { vi } from "vitest";
 
-export const stubTool = (name: string) => ({
+export type StubTool = {
+  name: string;
+  description: string;
+  parameters: { type: "object"; properties: Record<string, unknown> };
+  // Keep the exported type portable: don't leak Vitest's mock types into .d.ts.
+  execute: (...args: unknown[]) => unknown;
+};
+
+export const stubTool = (name: string): StubTool => ({
   name,
   description: `${name} stub`,
   parameters: { type: "object", properties: {} },
-  execute: vi.fn(),
+  execute: vi.fn() as unknown as (...args: unknown[]) => unknown,
 });
 
 vi.mock("../tools/image-tool.js", () => ({