[Attention][MLA] Re-enable FA4 as default MLA prefill backend#38819

LucasWilkinson merged 2 commits intomainvllm-project/vllm:mainfrom

revert-38562-fi_mla_prefill_defaultvllm-project/vllm:revert-38562-fi_mla_prefill_defaultCopy head branch name to clipboard