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当账号配置了模型映射（如 claude-sonnet-4-6 → glm-5.0）时，系统错误地
使用映射后的上游模型名计算费用。由于上游模型（如 glm-5.0）在定价系统中
没有价格配置，导致计费失败后被静默置为 0，用户不被扣费。

修改 forwardResultBillingModel 优先返回请求模型名，并移除 OpenAI 路径
中 BillingModel 字段对计费模型的覆盖逻辑。

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent

)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

Propagate UpstreamModel through ForwardResult and OpenAIForwardResult in Anthropic direct, API-key passthrough, Bedrock, and OpenAI gateway flows. Extract optionalNonEqualStringPtr and optionalTrimmedStringPtr into usage_log_helpers.go. Store upstream_model only when it differs from the requested model.

Also introduces anthropicPassthroughForwardInput struct to reduce parameter count.