Simplify search-based insight type: drop the runtime / backend split

Simplify search-based insight type: drop the runtime / backend split: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). While working on the code insights module I keep hitting friction caused by the way search-based insights are typed today. SearchBasedInsight is a…

The task

While working on the code insights module I keep hitting friction caused by the way search-based insights are typed today. `SearchBasedInsight` is a union of `SearchRuntimeBasedInsight` and `SearchBackendBasedInsight`, and the two variants have *different shapes*:

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.