[Login, Fetch user info] Unexpected error/FormatException occurs upon sending "Fetch user info" requests with invalid SSO sessions

bug major, reported by qa

**Describe the bug**
Unexpected error/FormatException occurs upon sending "Fetch user info" requests with invalid login sessions.

**Authgear Version**
- Version: Staging

**To Reproduce**
Steps to reproduce the behavior:
1. Log in to the project portal
2. Terminate the login sessions by 1) log out the SSO session from other devices, 2) terminate the session in the admin portal
3. Click the fetch user info API
4. See unexpected error / internal server error / format exception occurs on the demo app of Native / React Native / Flutter SDK

**Expected behavior**
Should show OAuthError or other invalid session error messages

**Screenshots**

**Client Env (if applicable, please complete the following information):**
- Device: Samsung s21 FE, iPhone 11 Pro
- OS: win 11, Android 12, iOS 16
- Browser: Chrome
- Browser Version: 108.0.5359.99

Start with the Fetch user info API flow in the Native, React Native, and Flutter demo apps and reproduce the request after terminating the SSO session. Trace how the invalid-session response is handled; done means the demos show OAuthError or another invalid-session message instead of an unexpected error, internal server error, or FormatException.

Written by the indexing model from the issue text.