fix: OpenAI 路径添加防御性 usage 合并逻辑

[Evsdrg]

Summary

将 OpenAI 路径中 message_delta 处理器的 { ...usage, ...deltaUsage }
spread 替换为 updateOpenAIUsage() 函数，与 claude.ts 的 updateUsage()
模式保持一致。

当前 OpenAI 适配器发送的 message_delta 包含完整字段，spread 不会丢失
数据。但若未来适配器在某事件中省略了 cache 字段（传 explicit 0），spread
会无声覆盖有效值。新函数仅在 delta 字段有意义值时更新，否则保留当前值。

Context

跨 provider 缓存兼容性分析（见 PR #1225 的后续讨论）发现：

• Anthropic 路径使用 updateUsage() 做字段级保护合���
• OpenAI 路径使用裸 { ...usage, ...deltaUsage } spread，缺少防御
• Codex/ChatGPT Responses API 路径同理（走同一个 message_delta 处理器）

Test plan

• tsc --noEmit 零错误
• biome format 零差异
• biome lint 零违规

🤖 Generated with Claude Code Best

Summary by CodeRabbit

• Bug Fixes
  • Improved token usage tracking for OpenAI API calls to accurately preserve cache-related metrics and prevent unintended resets of cached token counts.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 6d1e45f7-3285-4691-bdfd-0229040f06f9

📥 Commits

Reviewing files that changed from the base of the PR and between 5b941d4 and 59ab664.

📒 Files selected for processing (1)

• src/services/api/openai/index.ts

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📝 Walkthrough

Walkthrough

The OpenAI streaming adapter adds defensive merging logic for incremental token-usage updates. A new updateOpenAIUsage() helper preserves cache-related token fields (cache-creation and cache-read tokens) when streaming deltas provide zeros, preventing accidental zeroing of prior valid metrics. The helper is applied at the message_delta usage accumulation point, replacing a shallow object spread.

Changes

OpenAI Token Usage Defense

Layer / File(s)	Summary
Defensive usage merge helper and application src/services/api/openai/index.ts	Add updateOpenAIUsage() to selectively merge token deltas, updating input/output tokens directly while preserving cache token fields against zero overwrites. Apply the helper at message_delta accumulation to replace shallow spread.

Possibly related PRs

• claude-code-best/claude-code#226: Both PRs address OpenAI stream usage merging/mapping in the OpenAI adapter flow—specifically preserving/propagating message_delta token usage (including cache-related fields) and ensuring correct stop_reason/assembled AssistantMessage output.

Poem

A spreadsheet once flat could not hold its ground,
When zeros came crashing with nary a sound,
But updateOpenAIUsage() builds walls strong and true—
Cache tokens now safe from the overwrite crew! 🐰✨

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🎯 2 (Simple) | ⏱️ ~15 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title is in Chinese and describes the main change: adding defensive usage merge logic to the OpenAI path, which matches the file changes summary about implementing updateOpenAIUsage() helper for safer token-usage merging.
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.

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[Evsdrg]

Superseded by PR #1234 which extracts the same fix into a shared module (openaiShared.ts) used by both OpenAI and Grok paths.