feat(vllm): vllm gb200 dsv4 recipes

[alec-flowers]

Draft PR for the vLLM GB200 v0.20.0 branch.

Summary:

• Adds a self-contained lm-eval benchmark runner for GSM8K-style evals against an OpenAI-compatible chat endpoint.
• Keeps the existing SGLang-dependent gsm8k runner untouched; this new path uses python3 -m lm_eval --model local-chat-completions and does not require SGLang or an InferenceX workspace mount.
• Bundles the GSM8K task YAML, score thresholds, and score validator under src/srtctl/benchmarks/scripts/lm-eval/.
• Updates only recipes/vllm/deepseek-v4-pro/GB200/8k1k/disagg-gb200-1p4d-dep8-tp8-c256-c512-offload.yaml to run lm-eval with the bundled GSM8K task and VALIDATE_EVAL_SCORES=true.

Validation:

• Local smoke: launched a small Qwen/Qwen2.5-0.5B-Instruct OpenAI-compatible chat endpoint and ran the bundled script with EVAL_LIMIT=2, EVAL_NUM_FEWSHOT=0, EVAL_CONC=1; it produced meta_env.json, results_*.json, and samples_*.jsonl successfully.
• bash -n src/srtctl/benchmarks/scripts/lm-eval/bench.sh passed.
• python3 -m py_compile src/srtctl/benchmarks/lm_eval.py src/srtctl/cli/do_sweep.py src/srtctl/benchmarks/scripts/lm-eval/validate_scores.py passed.
• Focused tests passed: tests/test_benchmarks.py::TestLMEvalRunner, TestRunPostEval, and TestScriptsExist.
• UV_DEFAULT_INDEX=https://pypi.org/simple make check passed (635 passed, 2 skipped, 6 deselected).
• The known ty diagnostic in src/srtctl/core/validation.py is still emitted under the existing || true Makefile behavior.