"Login Failed: user sync failed" due to illegal base64 decode error in OAuth session

### What happened? ## What happened? ### Description We're observing intermittent login failures for users in Grafana v12.1.0, deployed on Kubernetes. Affected users see the following message in their browser after authenticating via OAuth: `Login Failed: user sync failed` In the Grafana logs, we see: ``` logger=user.sync t=2026-01-06T09:45:18.990501768Z level=error msg="Failed to fetch user" error="illegal base64 data at input byte 65532" auth_module=oauth_generic_oauth auth_id= ``` This appears to happen during the OAuth callback when session or state data is decoded. ### What did you expect to happen? Grafana should: Gracefully handle invalid session or state data. Fall back to a clean login if session restoration fails. Avoid user-facing failures due to internal decoding issues. ### Did this work before? Have been seeing the same issue since 11.3 version ### How do we reproduce it? Affects some users, not all. Users are unable to log in after successful OAuth authentication. No consistent pattern ### Is the bug inside a dashboard panel? NO ### Environment (with versions)? Grafana: 12.1.0 OS: Mac Browser: Google Chrome Browser : Version 143.0.7499.170 (Official Build) (arm64) ### Grafana platform? Kubernetes ### Datasource(s)? _No response_