Support partial rollout resume in Search-R1 example

[OLIVER-XYP]

Summary

• Enable partial rollout resume for the Search-R1 custom generate example.
• Preserve partial response, tokens, loss masks, and rollout log probabilities across aborted rollouts.
• Resume generation from existing Search-R1 context instead of restarting from the prompt.
• Document partial rollout usage in the Search-R1 READMEs.

Tests

• python -m py_compile examples\search-r1\generate_with_search.py tests\test_search_r1_partial_rollout.py
• pytest tests/test_sample.py tests/test_search_r1_partial_rollout.py -q