fix(vllm-omni): retire per-request AR seed patch

[leviking98z-rgb]

Summary

Remove the vLLM-Omni per-request AR seed monkey patch and its patch-table entry.

The patch replaced every unset SamplingParams.seed with os.urandom, based on
the assumption that reusing one SamplingParams object collapses a GRPO group to
identical tokens. vLLM 0.27 and 0.28 instead keep seed=None on the ordinary
random-sampling path, where the GPU RNG advances across batch rows. The override
is therefore unnecessary for UniRL's current single-replica recipes and makes
rollouts impossible to reproduce from the configured engine seed.

Related Issue

Refs #473.

Test Plan

• SKIP=no-commit-to-branch pre-commit run --all-files --show-diff-on-failure — passed.
• python -m compileall -q unirl/rollout/engine/vllm_omni/patches — passed.
• One-off sampler probe with vllm==0.28.0, vllm-omni==0.28.0,
  torch==2.13.0+cu130 on one NVIDIA H20: 64 seed=None rows produced 55
  distinct tokens, and resetting the Torch/CUDA seed reproduced the complete
  batch. No model checkpoint or dataset is involved in this sampler-level check.
• Source-checked vLLM v0.27.0: seed=None likewise avoids a per-request
  generator and uses the advancing batched GPU RNG.

Compatibility / Risk

Current UniRL vLLM-Omni recipes use one engine replica, so request diversity is
preserved by vLLM's advancing RNG. Independently initialized multi-replica
engines can still begin with correlated RNG streams; if that topology is added,
it should use deterministic seeds derived from request identity rather than host
entropy from os.urandom.

No config, checkpoint, data-format, or API migration is required.

Reviewer Notes

Open PR #413 removes the separate capture-flush direct registration as part of
the vLLM-Omni 0.28 migration, but retains this seed patch. No open PR was found
that removes or replaces the seed override.

AI assistance was used. The full two-file diff and all reported validation
output were reviewed before publication.

Checklist

• I reviewed the changed code and removed unrelated/generated artifacts.
• I updated tests, docs, and configs where needed, or explained why not.