feat(account-test): 增强 Sora 账号测试能力探测与弹窗交互

- 后端新增 Sora2 邀请码与剩余额度探测，并补充对应结果解析
- Sora 测试流程补齐请求头与 Cloudflare 场景提示，完善单测覆盖
- 前端测试弹窗对 Sora 账号改为免选模型流程，并新增中英文提示文案

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/i18n/locales/en.ts

@@ -1993,6 +1993,10 @@ export default {
       selectTestModel: 'Select Test Model',
       testModel: 'Test model',
       testPrompt: 'Prompt: "hi"',
+      soraTestHint: 'Sora test runs connectivity and capability checks (/backend/me, subscription, Sora2 invite and remaining quota).',
+      soraTestTarget: 'Target: Sora account capability',
+      soraTestMode: 'Mode: Connectivity + Capability checks',
+      soraTestingFlow: 'Running Sora connectivity and capability checks...',
       // Stats Modal
       viewStats: 'View Stats',
       usageStatistics: 'Usage Statistics',