fix: address review nits from #470

[Mec-iS]

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

The review of #470 left a few nits:

• RandomForestClassifier::predict_oob still used an is_none() guard before matching on self.classes, the same redundancy that was removed from Lasso.
• XGRegressor::predict matched on self.parameters but still called self.regressors.as_ref().unwrap(), which can panic if the two fields are out of sync after deserialization.
• DecisionTreeClassifier::predict and predict_proba use self.nodes.is_empty(), a third guard style, without a comment.
• The new should_not_panic tests do not carry cfg_attr(..., wasm_bindgen_test), so they do not run in the wasm CI job.
• The change-log table in the PR description of Decision tree panic #470 is inaccurate (KNNClassifier guards predict_proba, not predict_oob; several guarded methods were missing).
• The MultiClassSVC::predict error-text change from Decision tree panic #470 is not recorded in CHANGELOG.md.

New expected behaviour

• predict_oob folds both checks into one match (&self.trees, &self.classes), as Lasso does.
• XGRegressor::predict matches on (&self.parameters, &self.regressors); no unwrap remains.
• The nodes.is_empty() guards carry a short comment explaining why they are the right check.
• All 27 new should_not_panic tests carry the standard cfg_attr(all(target_arch = "wasm32", not(target_os = "wasi")), wasm_bindgen_test::wasm_bindgen_test) attribute.
• CHANGELOG.md gains an [Unreleased] section with the unfitted-predict fix and the breaking error-message note for MultiClassSVC.

Change logs

Fixed

• Unfitted-model guards: RandomForestClassifier::predict_oob and XGRegressor::predict no longer rely on redundant or panic-prone checks (predict method on DecisionTreeRegressor panics when tree is not fit #469 follow-up to Decision tree panic #470).

Changed

• Test metadata only: new should_not_panic tests now run under wasm-bindgen-test on wasm targets.
• CHANGELOG.md: document the Decision tree panic #470 behaviour change, including the new MultiClassSVC not-fitted error text.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-features -- -Drust-2018-idioms -Drust-2024-compatibility -Dwarnings
• cargo test --all-features