fix(devops): correct CompletionTokens assignment typo in callback handler

[SKDG042]

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fix

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CompletionTokens 被错误赋值成 PromptTokens

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en: In devops/internal/service/call_option.go:143, CompletionTokens was incorrectly assigned ext.TokenUsage.PromptTokens due to a copy-paste error. This causes the completion token count in DevOps debug metrics to always equal the prompt token count, making token usage statistics unreliable for billing analysis and performance monitoring.

zh(optional): 在 devops/internal/service/call_option.go:143 中，CompletionTokens 被错误地赋值为 ext.TokenUsage.PromptTokens。这导致 DevOps 调试指标中的 completion token 数量永远等于 prompt token 数量，使得 token 使用统计在计费分析和性能监控场景下不可信。

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