[MNT] remove imbalanced-learn test dependency and raise imbalance coverage

[TonyBagnall]

Address part of #3654.

After #3669 I will update this to remove the soft dependency, #3669 to go in first.

The SMOTE and ADASYN tests used imbalanced-learn as a runtime parity oracle behind a soft-dependency skip. Replace the skipped equivalence tests with hardcoded reference values captured from imblearn 0.14.2 (SMOTE k_neighbors=1 / ADASYN n_neighbors=1, random_state=49); this port reproduces them bit-identically, so the whole suite now passes with imbalanced-learn uninstalled.

While we are Raise imbalance package coverage from 89% to 99% with functional tests: multi-class skip of already-balanced classes, ADASYN RuntimeError (no majority neighbours) and ValueError (rounding yields no samples), ESMOTE determinism, and OHIT single-sample-minority tiling and no-cluster fallback.

Remove genuinely dead code rather than test it: the Borderline-SMOTE-2 y-weighting branch in SMOTE._generate_samples (never called; _make_samples always passes y=None) and the return_bias branch in ESMOTE (always called with return_bias=False).

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