ci: install matplotlib/seaborn for graphguard tests

[MeaFew]

Summary

• install the plotting/training deps used by the test suite: matplotlib, seaborn, xgboost
• run graph materialization under coverage so the coverage gate measures download_data / build_graph too

Why

tests/test_explain.py imports graphguard.explain_gnn, which imports matplotlib; the baseline training test imports xgboost. After those were fixed, the 30% coverage gate was still failing because the setup steps that execute download_data and build_graph ran outside coverage.

Test plan

• CI lint/test/typecheck all pass on the PR branch.