fix(slime): support rollout_id sample grouping

[YolandaLyj]

Summary

• construct Slime Samples with the trajectory-id field exposed by the installed revision
• prefer rollout_id for newer Slime revisions while retaining group_id compatibility for the v0.3.0 tag
• use the same compatibility order when grouping samples during reward post-processing
• document and test both schemas

Background

The Slime v0.3.0 tag renamed rollout_id to group_id, but this was reverted shortly afterward:

• rename to group_id: THUDM/slime@a3c5462
• restore rollout_id: THUDM/slime@44d29ee

As a result, the current bridge fails with TypeError: unexpected keyword argument 'group_id' when used with the Slime revision pinned by newer images/checkouts.

This checks the actual Sample dataclass fields instead of package metadata because image builds can report version 0.3.0 while containing the post-revert rollout_id schema.

Tests

29 passed in 0.43s

Also passed Ruff and git diff --check.