feat: add customizable pricing for native tools

[chenyme]

Summary

优化官方原生工具计费配置能力。

本次变更将所有官方原生工具统一为按次计费配置：管理员可以为每个原生工具单独设置 $xx / 次 价格，填 0 表示不生成单独工具账单项。后端统一归一化原生工具价格配置为 unit: "call"，并只在上游返回可解析的工具调用次数时生成原生工具账单明细，避免仅因模型开启工具就产生误扣费。

Change type

• Bug fix
• Feature
• Documentation
• Refactor
• Configuration / deployment
• Security hardening
• Other

Affected areas

• Frontend / UI
• Backend / API
• Authentication / authorization
• Conversations / streaming
• Files / RAG / extraction
• Model routing / providers
• MCP / tools
• Billing / payments
• Admin console
• Deployment / Docker / configuration
• Documentation

Verification

• cd backend && go test ./internal/application/billing ./internal/shared/nativetool ./internal/infra/llm

Screenshots, API examples, or logs

No screenshots attached.

Billing behavior:

• All official native tools display editable $xx / 次 pricing.
• 0 / 次 means no separate native tool billing item.
• Positive price is billed per upstream-reported native tool call.
• Tools without upstream-reported call counts do not generate separate native tool billing items.

Configuration, migration, and compatibility notes

• Native tool pricing is normalized to:
  • unit: "call"
  • priceLabel: ""
  • billable: priceNanousd > 0
• Existing saved native tool pricing configs are normalized on read/write.
• Default non-billable tools remain 0 / 次 unless an administrator sets a positive price.

Documentation

• Documentation is not needed for this change.
• Documentation was updated.
• Documentation still needs to be updated.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• User data access remains scoped by authenticated user context unless an admin-only path explicitly requires broader access.
• Security-sensitive behavior was reviewed, including billing and native tool usage accounting.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior, deployment steps, API contracts, or configuration changes are documented.
• Generated artifacts are included only when this project explicitly requires them.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.