test: add negative tests for isDocument and isWindow

Verify isDocument() and isWindow() return false when the globals are
undefined, simulating a service worker environment.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: joker23

packages/sdk/browser/__tests__/BrowserApi.test.ts

@@ -4,6 +4,26 @@ it('isDocument returns true when document is defined', () => {
   expect(isDocument()).toBe(true);
 });
 
+it('isDocument returns false when document is not defined', () => {
+  const original = Object.getOwnPropertyDescriptor(globalThis, 'document');
+  Object.defineProperty(globalThis, 'document', { value: undefined, configurable: true });
+  try {
+    expect(isDocument()).toBe(false);
+  } finally {
+    Object.defineProperty(globalThis, 'document', original!);
+  }
+});
+
 it('isWindow returns true when window is defined', () => {
   expect(isWindow()).toBe(true);
 });
+
+it('isWindow returns false when window is not defined', () => {
+  const original = Object.getOwnPropertyDescriptor(globalThis, 'window');
+  Object.defineProperty(globalThis, 'window', { value: undefined, configurable: true });
+  try {
+    expect(isWindow()).toBe(false);
+  } finally {
+    Object.defineProperty(globalThis, 'window', original!);
+  }
+});