Fix robust-mts-attack rolling test grouping

[kiwoongyoon]

Title: Fix robust-mts-attack rolling test grouping

Summary

• Split robust-mts-attack rolling test entries by evaluation date before stacking them, avoiding NumPy ragged-array conversion failures.
• Update the nursery grouper's date handling to work with current GluonTS Period starts.
• Add a targeted regression test for rolling test dates with different left-padded lengths.

Issue snapshot duplicate-work check: #3275 is unassigned, has no duplicate/blocking labels, and mentions no related PR.

Fixes #3275

Test evidence

• PYTHONPATH=src pytest -q test/nursery/robust_mts_attack/test_grouper.py passed: 1 passed in 0.01s
• git diff --check passed

Risks or notes for maintainers

• The change is limited to nursery robust-mts-attack code and one focused test.
• It preserves the existing assumption that rolling test entries are ordered by test date.
• Full traffic dataset training was not run because the issue did not include the exact command or dataset setup.