fix(rollout): forward top-k sampling configuration (#8676)
## Summary
`SamplingConfig` exposes `top_k`, but `HybridEngineRollout` did not
forward it to Transformers generation. As a result, sampling callers
could not apply their configured top-k limit and the default
non-positive setting could fall through to Transformers' own behavior.
This PR:
- forwards `top_k` when `HybridEngineRollout` uses random sampling;
- treats `top_k <= 0` as disabled by passing Transformers its disabled
value;
- preserves greedy generation, CUDA graph capture, and continuous
batching behavior;
- removes the unused local top-p sampler, since rollout sampling
delegates to Transformers.
## Validation
- verifies top-k forwarding during random sampling;
- verifies prompt expansion for multiple samples;
- verifies EOS padding and the `eos_token_id=None` ZeRO lockstep
contract;
- verifies disabled `top_k` values (`-1` and `0`);
- focused regression tests: 4 passed;
- Changed-file pre-commit checks: passed
Thank you for your review.
Signed-off-by: nathon-lee <leejianwoo@gmail.com>
