feat(openai): 支持自动透传开关并透传 User-Agent

- OpenAI OAuth/API Key 统一支持自动透传开关，编辑页可开关\n- 透传模式仅替换认证并保留计费/并发/审计，修复 API Key responses 端点拼接\n- Usage 页面显示原始 User-Agent 且不截断，补充回归测试与清单

frontend/src/i18n/locales/en.ts

@@ -1533,7 +1533,8 @@ export default {
         apiKeyHint: 'Your OpenAI API Key',
         oauthPassthrough: 'Auto passthrough (auth only)',
         oauthPassthroughDesc:
-          'When enabled, applies to Codex CLI requests only: the gateway forwards request/response as-is and only swaps OAuth auth, while keeping billing/concurrency/audit. Disable to rollback if you hit 4xx or compatibility issues.',
+          'When enabled, this OpenAI account uses automatic passthrough: the gateway forwards request/response as-is and only swaps auth, while keeping billing/concurrency/audit and necessary safety filtering.',
+        modelRestrictionDisabledByPassthrough: 'Automatic passthrough is enabled: model whitelist/mapping will not take effect.',
         enableSora: 'Enable Sora simultaneously',
         enableSoraHint: 'Sora uses the same OpenAI account. Enable to create Sora account simultaneously.'
       },