refactor(quickadapter): return cached sets directly in optuna_samplers_by_namespace
- Add _optuna_hpo_samplers_set() and _optuna_label_samplers_set() cached methods
- Change return type from tuple[tuple[OptunaSampler, ...], OptunaSampler] to tuple[set[OptunaSampler], OptunaSampler]
- Remove redundant set() conversion in sampler validation
- Align with existing pattern used by other constant set methods (_scaler_types_set, _threshold_methods_set, etc.)