Add support for vllm >= 0.19.0 (#1211)

* Fix vLLM 0.11 compatibility and restore hellaswag_cf

Co-authored-by: OpenAI Codex <codex@openai.com>

* Support vLLM 0.19 prompt schema

Co-authored-by: OpenAI Codex <codex@openai.com>

* Address vLLM PR review feedback

Co-authored-by: OpenAI Codex <codex@openai.com>

* Remove temporary hellaswag_cf task

Co-authored-by: OpenAI Codex <codex@openai.com>

* Clarify vLLM compatibility branches

Co-authored-by: OpenAI Codex <codex@openai.com>

* Handle tied MCQ logits in slow sample comparisons

Co-authored-by: OpenAI Codex <codex@openai.com>

* Handle flat VLM token outputs in tie checks

Co-authored-by: OpenAI Codex <codex@openai.com>

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Co-authored-by: OpenAI Codex <codex@openai.com>