fix: improve TypeScript types for better IDE code hinting

- Install @types/bun and @types/jsdom for proper type definitions
- Fix mock function typing in test files
- Create jest-dom.d.ts to properly extend Bun's Matchers interface
- Remove MockedFunction imports (not supported in Bun's types)
- Cast global.fetch and useSession mocks to any where needed
- Fix mock.module return types to match expected signatures

This resolves TypeScript errors that were preventing proper code
completion and type checking in IDEs.

src/__tests__/auth-simple.test.ts

@@ -1,19 +1,19 @@
-import { describe, it, expect, mock, beforeEach, MockedFunction } from 'bun:test'
+import { describe, it, expect, mock, beforeEach } from 'bun:test'
 import { getSession } from '@/lib/auth-simple'
 
 // Mock next/headers
 mock.module('next/headers', () => ({
-  cookies: mock(() => {}).mockResolvedValue({
-    get: mock(() => {}).mockReturnValue({ name: 'better-auth.session_token', value: 'test-token' }),
-  }),
-  headers: mock(() => {}).mockResolvedValue({
-    get: mock(() => {}).mockReturnValue(null),
-  }),
+  cookies: mock(() => Promise.resolve({
+    get: mock(() => ({ name: 'better-auth.session_token', value: 'test-token' })),
+  })),
+  headers: mock(() => Promise.resolve({
+    get: mock(() => null),
+  })),
 }))
 
 // Mock fetch
-const mockFetch = mock(() => {})
-global.fetch = mockFetch as MockedFunction<typeof global.fetch>
+const mockFetch = mock(async () => new Response())
+global.fetch = mockFetch as any
 
 describe('getSession', () => {
   beforeEach(() => {