test(tree): pin min_samples_split boundary and assert depth after #464

[Mec-iS]

Fixes #465

Checklist

• My branch is up-to-date with main branch.
• Everything works and tested on latest stable Rust.
• Coverage and Linting have been applied

Current behaviour

Follow-up to #464 (off-by-one fix in the tree growth loop). Review feedback left two coverage gaps:

1. No regression test pins the min_samples_split boundary. A node of exactly 3 samples with min_samples_split: 3 must still split under the fixed n >= min_samples_split rule; with min_samples_split: 4 it must stay a single leaf.
2. The full_depth test asserts nodes().len() == 7 and accuracy but never tree.depth(), although DecisionTreeClassifier::depth() is public API and the growth loop no longer drives the depth counter directly.

New expected behaviour

• min_samples_split_boundary tests (classifier and base regressor): min_samples_split: 3 on a 3-sample node yields 3 nodes and depth == 2; min_samples_split: 4 yields 1 node and depth == 0.
• full_depth tests now also assert depth == 3 for both the classifier (tree.depth(), public API) and the base regressor.
• Patch version bumped to 0.6.15 with a CHANGELOG.md entry. Library code is unchanged.

Change logs

Fixed

• tree: regression tests pinning the min_samples_split boundary and the tree depth after the Fix off-by-one in tree growth loop for DecisionTreeClassifier and BaseTreeRegressor #464 growth-loop fix.