Describe the bug
In residuals.py, _fit_residual_estimator has a return type of Tuple[NDArray, NDArray] but actually returns a RegressorMixin.

This isn't caught by mypy because ignore_missing_imports = True causes sklearn's RegressorMixin to be treated as Any, which is compatible with any return type.

Suggested fix
Add warn_return_any = True to mypy.ini to catch cases where Any is returned from typed functions.

To Reproduce
Steps to reproduce the behavior:
uv run make type-check

Expected behavior
I expect to have an error being raised when such a typing error is present.