Stage 1: Fill untested substantive files + revive disabled tests (tracking #391)

[Mec-iS]

Tracking issue: #391 — Improve test coverage significantly (staged plan).

Scope

Fill the untested substantive files and revive disabled tests. Highest-ROI stage.

Tasks

• src/linalg/traits/high_order.rs: implement the empty /* TODO: Add tests */ mod tests. Cover all 4 transpose-flag branches of ab ((true,true), (false,true), (true,false), (false,false)) and non-square inputs.
• src/linear/lasso_optimizer.rs: add direct tests for InteriorPointOptimizer (currently only exercised transitively via lasso.rs::lasso_fit_predict). Cover new, optimize on a small l1-regularized least-squares problem, convergence on a known-answer input.
• src/error/mod.rs: tests for all Failed constructors (fit, predict, transform, because), every FailedError variant, Display formatting, PartialEq.
• src/api.rs: trait-coverage tests for UnsupervisedEstimator / SupervisedEstimator / Predictor / Transformer / NoParameters wiring where behavior is testable.
• src/rand_custom.rs: RNG determinism tests across std_rand (feature) vs SmallRng paths, and seed reproducibility.
• Revive the 13 commented-out #[test]s across 12 files (mostly disabled serde round-trip tests in decomposition/pca.rs, decomposition/svd.rs, svm/svr.rs, linear/*, dataset/*, svm/search/svc_params.rs). Gate them with #[cfg(feature = "serde")].
• Fix mis-named tests:
  • src/dataset/diabetes.rs::boston_dataset → diabetes_dataset
  • src/algorithm/sort/quick_sort.rs::with_capacity → quick_argsort (copy-paste from heap_select.rs)

Verification

• cargo test --all-features
• cargo fmt --all -- --check
• cargo clippy --all-features -- -Drust-2018-idioms -Dwarnings

Do you want to work on this issue?

yes — PR will target development.