Fix NumericImputationConfig block-model overrides: knn_n_neighbors and mice_max_iter

[DEVunderdog]

Parent

#92

What to build

Correct the config surface for KNN and MICE block-model overrides. Both strategies fit one shared model for the entire block — per-column dict overrides are semantically wrong. See ADR-0037.

Remove per_column_n_neighbors: dict[str, int]. Add knn_n_neighbors: Optional[int] — scalar, KNN block only. All KNN columns share one KNNImputer; a single value governs the entire block.

Add mice_max_iter: Optional[int] — scalar, MICE block only. All MICE columns share one IterativeImputer; a single value governs the entire block.

Retain per_column_max_iter: dict[str, int] for Regression only — each Regression column gets its own IterativeImputer, so per-column values are semantically valid. Update its docstring to say Regression only (remove the MICE mention).

Acceptance criteria

• per_column_n_neighbors field removed from NumericImputationConfig
• knn_n_neighbors: Optional[int] = None added to NumericImputationConfig
• mice_max_iter: Optional[int] = None added to NumericImputationConfig
• per_column_max_iter docstring updated to say Regression columns only
• knn_n_neighbors and mice_max_iter appear in to_dict() output
• from_dict() round-trips both fields correctly; from_dict({}) produces None defaults
• test_imputation_config.py: existing per_column_n_neighbors tests replaced with knn_n_neighbors equivalents; mice_max_iter default, to_dict, and round-trip tests added
• Numpy-style docstring on NumericImputationConfig updated for all three changed fields (per ADR-0034)

Blocked by

None — can start immediately