fix: add conversations alias in test data for backward compatibility

calibrate_draft_vocab.py and compute_hidden_states_hf.py still read
the conversations field. Include both messages and conversations in
test data until those scripts are updated.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Chenhan Yu <chenhany@nvidia.com>

Author: ChenhanYu