test: add unit tests for MortalityPredictionMIMIC3 and DrugRecommendationMIMIC3

[haoyu-haoyu]

Summary

• Add data-driven unit tests for MortalityPredictionMIMIC3 and DrugRecommendationMIMIC3
• Tests follow the established pattern in test_mimic3_readmission_prediction.py
• Uses existing MIMIC-III demo data — no new test fixtures needed

Tests added

test_mimic3_mortality_prediction.py (8 tests)

Test	What it verifies
test_task_schema	task_name, input/output schema keys
test_sample_schema	Required keys in each sample
test_mortality_label_is_binary	Labels are 0 or 1
test_mortality_label_from_next_visit	Patient 10059 visit 142582 → mortality=1 (next visit expired)
test_surviving_next_visit	Patient 10119 visit 157466 → mortality=0
test_last_visit_excluded	Last admission has no future visit to label
test_single_visit_patients_excluded	Patient 10006 (1 visit) produces no samples
test_visit_without_procedures_excluded	Patient 10117 visit 187023 (no procs/rx) skipped

test_mimic3_drug_recommendation.py (6 tests)

Test	What it verifies
test_task_schema	nested_sequence inputs, multilabel output
test_sample_schema	Required keys including visit_id, drugs_hist
test_conditions_are_nested	2-D tensor (nested_sequence)
test_single_visit_patients_excluded	Patient 10006 excluded
test_visit_without_procedures_excluded	Visit 118192 (no procs) excluded
test_multi_visit_patient_produces_samples	Patient 10088 (3 visits) produces samples

Test plan

• CI passes (pytest, linting)
• No overlap with PR test: add unit tests for EmbeddingModel, GAN, metrics, and label processors #956 (which covers metrics + label processors)

Relates to #425, #338