test(core): mock fetch in OAuth transport fallback tests (#17059)

Co-authored-by: Bryan Morgan <bryanmorgan@google.com>

packages/core/src/tools/mcp-client.test.ts

@@ -1998,6 +1998,19 @@ describe('connectToMcpServer - OAuth with transport fallback', () => {
     vi.spyOn(console, 'warn').mockImplementation(() => {});
     vi.spyOn(console, 'error').mockImplementation(() => {});
 
+    // Mock fetch to prevent real network calls during OAuth discovery fallback.
+    // When a 401 error lacks a www-authenticate header, the code attempts to
+    // fetch the header directly from the server, which would hang without this mock.
+    vi.stubGlobal(
+      'fetch',
+      vi.fn().mockResolvedValue({
+        status: 401,
+        headers: new Headers({
+          'www-authenticate': `Bearer realm="test", resource_metadata="http://test-server/.well-known/oauth-protected-resource"`,
+        }),
+      }),
+    );
+
     mockTokenStorage = {
       getCredentials: vi.fn().mockResolvedValue({ clientId: 'test-client' }),
     } as unknown as MCPOAuthTokenStorage;
@@ -2019,6 +2032,7 @@ describe('connectToMcpServer - OAuth with transport fallback', () => {
 
   afterEach(() => {
     vi.clearAllMocks();
+    vi.unstubAllGlobals();
   });
 
   it('should handle HTTP 404 → SSE 401 → OAuth → SSE+OAuth succeeds', async () => {