Auth: Backend datasources are giving 401 after 30m

### What happened? We have an SSO based generic oauth configured on our Grafana. We are on v11.2 currently and it's working fine. I tried upgrading to v11.6.0 and it works fine at the beginning but, after 30 minutes of usage, I see a 401 unauthorized from backend datasources. I debugged this further and looks like backend is receiving empty token. ### What did you expect to happen? Backend should receive proper oauth token instead of empty ### Did this work before? Yes ### How do we reproduce it? 1. Upgrade to v11.6 2. Open explore and execute a query with PromQL datasource that requires backend authentication 3. Wait for 30m and reload the query, it shows 401 ### Is the bug inside a dashboard panel? No ### Environment (with versions)? Grafana: v11.6.0 OS: Mac OS Browser: Safari ### Grafana platform? None ### Datasource(s)? _No response_