Dashboards (live): mixed data sources with a live underlying source crashes the panel

### What happened? https://github.com/user-attachments/assets/74bc2b4d-b4dc-4b04-9a1a-2dc1ca0fe6eb ### What did you expect to happen? I expected that the Mixed data source would work, and would allow one to combine historical and live streaming data in a single visualization. ### Did this work before? I have no idea. ### How do we reproduce it? 1.Set up an mqtt data source; 2.push some test data to a test mqtt topic; 3.create a panel that's using mqtt source and that topic; 4.observe it still working; 5.change data source for the panel to mixed; 6.observe it having completely hung. ### Is the bug inside a dashboard panel? _No response_ ### Environment (with versions)? Grafana: v12.2.0 OS: NixOS Browser: Firefox ### Grafana platform? A package manager (APT, YUM, BREW, etc.) ### Datasource(s)? MQTT