feat: expose video task billing usage (token via header, seconds in body)

[feitianbubu]

📝 变更描述 / Description

向视频任务查询接口的输出补充计费计量信息，方便下游（含中转/网关场景）据此计费。分两类：

1. 按量（token）计费 → 响应头 X-New-Api-Usage

在 GET /v1/videos/{task_id} 响应上新增响应头，携带任务 token 用量：

X-New-Api-Usage: {"completion_tokens": 1234, "total_tokens": 1234}

• 仅在适配器 ParseTaskResult 能解析出 total_tokens > 0 时设置。
• 走响应头而非响应体，body 保持纯 OpenAI 视频对象标准结构。

2. 按秒计费 → 响应体标准字段 seconds

ToOpenAIVideo 统一将计费所用时长（BillingContext.OtherRatios["seconds"]，即实际计费依据）写入标准的 seconds 字段：

{ "model": "...", "status": "completed", "seconds": "5", ... }

• seconds 是 OpenAI 视频对象的标准字段，不污染结构。
• 单点实现，所有按秒计费渠道（Ali/Veo 等）自动生效；非按秒渠道（无 seconds 倍率）不受影响。
• 返回值等于实际计费依据，显示与扣费一致。

两处均为纯增量，不改变任何现有计费/响应逻辑；普通客户端忽略即可。

🚀 变更类型 / Type of change

• ✨ 新功能 (New feature)

🔗 关联任务 / Related Issue

• N/A

✅ 提交前检查项 / Checklist

• 人工确认: 描述由人工整理撰写。
• 非重复提交: 已确认无重复 PR。
• 变更理解: 已理解改动原理与影响范围。
• 范围聚焦: 仅向 video 响应补充计量信息（header + seconds 字段），无其它改动。
• 本地验证: go build ./... 通过。
• 安全合规: 无敏感凭据，仅暴露计量信息，符合代码规范。

📸 运行证明 / Proof of Work

• 按量模型完成态查询：响应头 X-New-Api-Usage: {"completion_tokens":1234,"total_tokens":1234}。
• 按秒模型（如 happyhorse / veo）完成态查询：响应体 "seconds": "5"，与实际计费时长一致。

Summary by CodeRabbit

• New Features
  • Added support for exporting token usage metadata on video fetch responses when available.
  • Video fetch responses now include a usage header alongside the standard OpenAI-compatible video payload.
  • Extended OpenAI video output to optionally include per-second billing duration when provided by billing context.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 81a68c7d-8f4a-4721-9acd-cd8b7ca17939

📥 Commits

Reviewing files that changed from the base of the PR and between 969ecca and 9a8459b.

📒 Files selected for processing (1)

• model/task.go

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Walkthrough

The OpenAI video response path now adds upstream usage data to a response header and includes billing-derived seconds in the converted video payload when available.

Changes

Video response enrichment

Layer / File(s)	Summary
Parse task result and set usage header relay/relay_task.go	Parses the upstream task result, marshals token usage when totals are present, and sets the X-New-Api-Usage response header.
Populate video seconds from billing context model/task.go	Adds strconv import support and sets openAIVideo.Seconds from the billing context seconds ratio when it is positive.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• QuantumNous/new-api#1659: Both PRs modify videoFetchByIDRespBodyBuilder in relay/relay_task.go along the OpenAI video conversion path.
• QuantumNous/new-api#2141: Adds ParseTaskResult/upstream task result plumbing that aligns with this PR’s usage-header logic.

Poem

A bunny peeked at video streams,
Counted tokens, stitched sweet dreams,
Seconds hopped in, neat and bright,
Headers gleamed with usage light,
Thump! The payload feels just right.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main changes: exposing billing usage via a header and seconds in the video body.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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