Harden the public helper/fields configuration-field API so validation is safe, complete, and consistent…

Harden the public helper/fields configuration-field API so validation is safe, complete, and consistent…: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). Preserve the getter contract while making validation stricter. FieldData.Get returns an explicitly supplied…