MergeStructs / MergeStructInto should support overwriting destination values

MergeStructs / MergeStructInto should support overwriting destination values: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). I'm using ygot.MergeStructs (and MergeStructInto ) to combine two ValidatedGoStruct s of the same type — think a baseline config plus an…

The task

I'm using `ygot.MergeStructs` (and `MergeStructInto`) to combine two `ValidatedGoStruct`s of the same type — think a baseline config plus an override layer that should "win" where it sets something.

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