fix(binning): make get_feature_names_out follow the sklearn contract

[ChrisW09]

Fixes #36.

Problem

CustomBinTransformer.get_feature_names_out() raised when called with no arguments and
returned a plain list when given them. Both break scikit-learn's contract, and the first
breaks Pipeline.get_feature_names_out() for any pipeline containing the transformer:

t = CustomBinTransformer(output_dim=4).fit(X)
t.get_feature_names_out()
# InvalidParamError: input_features must be specified

Pipeline([("bin", CustomBinTransformer(output_dim=4))]).fit(X).get_feature_names_out()
# InvalidParamError: input_features must be specified

The package had already fixed exactly this for three sibling transformers — the docstring of
tests/test_feature_names_out.py records it as "get_feature_names_out(None) defaults to
generated x0, x1, ... names instead of raising, for NoTransformer, ToFloatTransformer
and ContinuousOrdinalTransformer" — but CustomBinTransformer was missed.

Fix

Mirror NoTransformer.get_feature_names_out: generate x0, x1, ... when input_features is
None, always return an ndarray, and guard on being fitted (which the old version did not do
at all).

no args    -> ['x0'] | ndarray
with names -> ['f']  | ndarray
Pipeline   -> ['x0']

Changed existing expectations

Both flagged in the issue:

• test_custom_bin_transformer_feature_names_out asserted names == ["feature1"], which
  only holds for a list. Now checks the ndarray type and its contents.
• test_custom_bin_transformer_feature_names_out_raises asserted the no-argument call
  raises — i.e. it pinned the defect. Replaced with
  test_custom_bin_transformer_feature_names_out_before_fit_raises, checking the
  NotFittedError that the new check_is_fitted guard produces, which is the assertion that
  should have been there.

Tests

Three added beyond those: generated default names, the pipeline call, and
len(get_feature_names_out()) == transform(X).shape[1].

Full suite: 483 passed, 9 xfailed. ruff check pretab clean apart from the pre-existing
RUF100; the remaining I001/B017/RUF043 in this test file all predate the change.
pyright unchanged at 71.

🤖 Generated with Claude Code