Unify DART SHAP forwarding

[RAMitchell]

This PR is another small step toward the follow-up work to remove GBDart as a separate booster and treat DART as weighted GBTree prediction/training behavior. It removes the DART-specific SHAP forwarding overrides and routes contribution and interaction prediction through the shared GBTree path, with DART supplying tree weights through a shared accessor. The scope here is intentionally limited to SHAP forwarding; serialization, training behavior, and booster identity are unchanged. This is groundwork for the larger PR that removes GBDart.

[Copilot]

Pull request overview

This PR is groundwork toward treating DART as a weighted GBTree by removing DART-specific SHAP forwarding overrides and routing SHAP contribution/interaction prediction through the shared GBTree path, with DART supplying per-tree weights via a shared accessor.

Changes:

• Add a GBTree::TreeWeights() virtual accessor and pass it into predictor contribution/interaction APIs.
• Remove DART overrides for SHAP forwarding and instead provide DART weights via TreeWeights().
• Add CPU/GPU tests that exercise SHAP contributions/interactions for a DART-trained model.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/cpp/predictor/test_shap.h	Exposes CheckDartShapOutput helper for both CPU and GPU test TUs.
tests/cpp/predictor/test_shap.cc	Implements CheckDartShapOutput and adds a CPU unit test for DART SHAP additivity.
tests/cpp/predictor/test_shap.cu	Adds a GPU unit test for DART SHAP additivity.
src/gbm/gbtree.h	Routes SHAP contribution/interaction prediction through shared path and passes TreeWeights().
src/gbm/gbtree.cc	Removes DART SHAP forwarding overrides; supplies DART weights through TreeWeights().

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.