[Rollout] Add opt-in group-scoped session affinity

[chengcuiping]

Summary

• Add an opt-in group scope for automatically assigned rollout session IDs.
• Preserve the existing per-sample behavior by default.
• Reuse the existing X-SMG-Routing-Key path for consistent-hashing routing.

Motivation

• Slime already preserves session affinity across turns of one sample.
• Grouped sibling rollouts may also need a shared routing identity.
• Previously this required callers to pre-populate the same session_id manually.

Behavior

• sample: unchanged default, one generated ID per sample.
• group: one generated or inherited ID per rollout group.
• conflicting explicit IDs fail closed.
• group scope requires consistent_hashing.
• non-consistent-hashing routing remains unchanged.

Testing

CPU-only qualification was run against the latest upstream main after cherry-picking this PR.

Additional production-path coverage was added for:

• generate_and_rm_group -> generate -> HTTP post
• verifying that 3 sibling requests from the same group carry the same X-SMG-Routing-Key
• preserving the default per-sample routing behavior for non-grouped generation

Commands:

CUDA_VISIBLE_DEVICES='' python -m compileall -q slime/rollout/sglang_rollout.py slime/utils/arguments.py tests/test_group_session_affinity.py tests/test_megatron_argument_validation.py
CUDA_VISIBLE_DEVICES='' python -m pytest tests/test_group_session_affinity.py tests/test_sample.py tests/test_megatron_argument_validation.py -m unit
CUDA_VISIBLE_DEVICES='' pre-commit run --files slime/utils/arguments.py slime/rollout/sglang_rollout.py tests/test_megatron_argument_validation.py tests/test_group_session_affinity.py docs/en/advanced/sglang-config.md docs/zh/advanced/sglang-config.md

Qualification result: 31 tests passed. git diff --check passed. Testing was CPU-only; no GPU was used.

Limitations

• This controls routing identity only.
• It does not implement KV pinning, eviction, transfer, TTL, or cache management.
• It does not guarantee a cache hit or claim a throughput or other performance improvement.
• Group affinity may reduce cross-worker load-balancing flexibility.

Related work

Related to #1469.
Builds on the consistent-hashing routing path introduced by #1588.