fix: Replace jest.fn() with vi.fn() in auth tests

The three RequestInit tests were incorrectly using jest.fn() instead of
vi.fn(), causing CI failures. The rest of the test file uses Vitest.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: dsp-ant

src/client/auth.test.ts

@@ -2563,7 +2563,7 @@ describe('OAuth Authorization', () => {
         it('custom headers from RequestInit are passed to auth discovery requests', async () => {
             const { createFetchWithInit } = await import('../shared/transport.js');
 
-            const customFetch = jest.fn().mockResolvedValue({
+            const customFetch = vi.fn().mockResolvedValue({
                 ok: true,
                 status: 200,
                 json: async () => ({
@@ -2596,7 +2596,7 @@ describe('OAuth Authorization', () => {
         it('auth-specific headers override base headers from RequestInit', async () => {
             const { createFetchWithInit } = await import('../shared/transport.js');
 
-            const customFetch = jest.fn().mockResolvedValue({
+            const customFetch = vi.fn().mockResolvedValue({
                 ok: true,
                 status: 200,
                 json: async () => ({
@@ -2634,7 +2634,7 @@ describe('OAuth Authorization', () => {
         it('other RequestInit options are passed through', async () => {
             const { createFetchWithInit } = await import('../shared/transport.js');
 
-            const customFetch = jest.fn().mockResolvedValue({
+            const customFetch = vi.fn().mockResolvedValue({
                 ok: true,
                 status: 200,
                 json: async () => ({