feat(core-testing): scaffold shared testing utilities package

Adds @repo/core-testing (tag: tooling) with:
- factory/defineFactory: monotonic-sequence object factories with overrides
- contract/defineContractSuite: shared test suites runnable against multiple impls
- react/renderWithProviders + createMockTrpcClient: RTL helpers
- payload/stubPayloadConfig + mockPayloadModule: Payload mocking helpers
- setup/{jsdom,node}: vitest setup files

Spec: docs/superpowers/specs/2026-05-05-tdd-foundation-design.md §5

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-testing/src/react/render-with-providers.tsx

@@ -0,0 +1,29 @@
+import type { PropsWithChildren, ReactElement } from "react";
+import { render } from "@testing-library/react";
+import type { RenderResult } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+
+export interface RenderOptions {
+  queryClient?: QueryClient;
+}
+
+export function renderWithProviders(
+  ui: ReactElement,
+  options: RenderOptions = {},
+): RenderResult & { queryClient: QueryClient } {
+  const queryClient =
+    options.queryClient ??
+    new QueryClient({
+      defaultOptions: {
+        queries: { retry: false },
+        mutations: { retry: false },
+      },
+    });
+
+  const Wrapper = ({ children }: PropsWithChildren) => (
+    <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+  );
+
+  const result = render(ui, { wrapper: Wrapper });
+  return Object.assign(result, { queryClient });
+}