fix(dsv4): gate h200 reasoning parser flag

[OCWC22]

Summary

• gate the H200 DSv4 --reasoning-parser deepseek_v4 flag behind ENABLE_DSV4_REASONING_PARSER=true
• keep the parser disabled by default because the AgentX testing branch's docs/AGENTIC_TEST_RESULTS.md records dsv4-fp8-h200-vllm as ❌ FAILED with the DeepSeek-V4 reasoning parser issue (reasoning_start_str / reasoning_end_str missing)
• preserve an opt-in path for future vLLM parser fixes without changing the default launcher behavior

Smoke test

• bash -n benchmarks/single_node/dsv4_fp8_h200.sh passes
• default ENABLE_DSV4_REASONING_PARSER expansion leaves REASONING_PARSER_ARGS empty, so the launcher omits --reasoning-parser deepseek_v4 unless explicitly enabled

[claude]

Claude Code Review

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[Copilot]

Pull request overview

This PR keeps the DeepSeek-V4 H200 single-node launcher stable by disabling the vLLM DeepSeek-V4 reasoning parser by default, while preserving an opt-in path via an environment flag.

Changes:

• Introduces ENABLE_DSV4_REASONING_PARSER (default false) to control whether --reasoning-parser deepseek_v4 is passed to vllm serve.
• Refactors the reasoning-parser CLI arg into an optional Bash array so the argument is omitted cleanly when disabled.

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