server: PEG chat parser returns HTTP 500 on complete generations containing invalid UTF-8 (no content fallback, unlike legacy parser)

当 llama.cpp 服务端模型在一次完整生成中输出了非法 UTF-8 字节(例如 OCR/视觉模型或字节回退 token 产生的乱码),PEG 聊天解析器会抛出异常并导致 /chat/completions 返回 HTTP 500;优先确认是否是 PEG 解析路径在完整生成上失败,而同样的内容在







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