refactor of group 3 (sampling): complete Group 3 code review for data loading,...

refactor of group 3 (sampling): complete Group 3 code review for data loading, checkpointing, and exception handling

- Introduce PyAETNAConfigError exception hierarchy in engine/exceptions.py to replace sys.exit calls in data_loader.py
- Optimize data_loader.py imports, add warning logs for unmapped visits, and improve type hints
- Refactor PlData in data.py: remove dead EXPTIME branch, add type hints, and implement __repr__
- Enhance checkpoint.py robustness:
  - Add JAX x64 early-return guard to upcast_value_to_float64 and support recursive list upcasting
  - Store samples_shapes in save_pickle and prioritize shape-based chain count recovery on load
  - Fix fragile array truthiness checks and avoid config dict mutation
  - Remove redundant docstring in save_pickle
- Add diagnostic ValueError guards for unmapped visit_to_inst_id entries in model/detrending.py
- Add comprehensive test suite in tests/test_data_loading.py (48 passing unit and integration tests)
- Update localDocs/code_review_plan_group3.md to document checklist completion and resolved open points