backfillReasoningInMessages

为 assistant 消息回填缺失的 reasoning_content（由调用方提供 lookup）。

函数签名

type OpenAiWireMessage = Record<string, unknown>

function backfillReasoningInMessages(
  messages: OpenAiWireMessage[],
  lookup: (message, priorMessages) => string | null,
): OpenAiWireMessage[]