Add E2E tests for OAuth authentication flows and provider integrations

- Implemented comprehensive E2E tests for OAuth buttons on login and register pages, including Google and GitHub provider interactions.
- Verified OAuth provider buttons' visibility, icons, and proper API integration with mock endpoints.
- Added button interaction tests to ensure correct API calls for authorization and state handling.
- Updated `playwright.config.ts` to include the new `auth-oauth.spec.ts` in test configurations.
- Extended mock handlers in `overrides.ts` and `auth.ts` to support OAuth-specific API workflows and demo scenarios.

frontend/playwright.config.ts

@@ -111,6 +111,7 @@ export default defineConfig({
         /auth-register\.spec\.ts/,
         /auth-password-reset\.spec\.ts/,
         /auth-flows\.spec\.ts/,
+        /auth-oauth\.spec\.ts/,
         /theme-toggle\.spec\.ts/,
       ],
       use: { ...devices['Desktop Chrome'] },