Update part of the AI policy

Partial update. Omitted fields keep their current value, and inside the override maps (model_substitutions, default_models, model_classes, operation_classes, features) omitted keys survive too. Sending null as a map value removes that single override and restores the platform default for it.

status defaults to the policy's current status (or active when none is stored), so patching an active policy keeps it active and the satisfiability gate binds exactly as it does on PUT. The same camelCase rule applies to values inside model_classes and to features keys.