Implement FabAuthManager.refresh_user for sliding JWT refresh

[roshan-ramees]

What?

Implements FabAuthManager.refresh_user so JWTRefreshMiddleware reissues the _token cookie on every request when the user is authenticated.

Why?

JWTRefreshMiddleware (added in #55506, 3.1.4) drives sliding-refresh of the UI JWT by calling auth_manager.refresh_user(user). BaseAuthManager.refresh_user is a no-op by default. KeycloakAuthManager overrides it — FabAuthManager does not. On FAB deployments (the default), the silent-refresh path does nothing, so long-open browser tabs burst 401s from concurrent /ui/* and /api/v2/* polls every [api_auth] jwt_expiration_time interval.

Symptom on our prod (Airflow 3.1.7 + FAB + Google OAuth):

[error] JWT token is not valid: Signature has expired
  [airflow.api_fastapi.auth.managers.base_auth_manager] loc=base_auth_manager.py:107
INFO: - "GET /ui/dashboard/dag_stats HTTP/1.1" 401 Unauthorized
INFO: - "GET /ui/dags?dag_runs_limit=14&limit=50&offset=0&exclude_stale=true&order_by=dag_display_name HTTP/1.1" 401 Unauthorized
INFO: - "GET /api/v2/dags/~/dagRuns/~/hitlDetails?state=deferred&response_received=false HTTP/1.1" 401 Unauthorized

Related open issue: #57065.

How?

Return the current user for authenticated (non-anonymous) FAB sessions. deserialize_user above already re-fetches the user from DB by token["sub"], so the object is fresh — returning it is safe and adds only one HS512 sign per request (sub-ms).

For anonymous / no user, return None to skip the reissue.

Tests

Three unit tests covering: authenticated user → returns user; anonymous user → returns None; no user → returns None.

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