test(fetch-client): restore globalThis.fetch in afterEach

Save original fetch before mocking and restore it after each test to
prevent cross-test pollution.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ymc9

packages/clients/fetch-client/test/fetch-client.test.ts

@@ -18,13 +18,15 @@ function makeSerializedResponseText(data: unknown) {
 
 describe('createClient', () => {
     let mockFetch: ReturnType<typeof vi.fn>;
+    const originalFetch = globalThis.fetch;
 
     beforeEach(() => {
         mockFetch = vi.fn();
         globalThis.fetch = mockFetch as unknown as typeof globalThis.fetch;
     });
 
     afterEach(() => {
+        globalThis.fetch = originalFetch;
         vi.resetAllMocks();
     });