Add SessionProxyPool for per-session sticky proxy allocation with
load-balanced least-used selection and auto-rotation on failure.
Introduce fetchWithSessionRetry for transparent retry with proxy
rotation. Wire into AI proxy handlers (OpenAI + Anthropic) with
stream lifecycle cleanup.
Co-Authored-By: Claude <noreply@anthropic.com>
Introduce a centralized `fetch-utils.ts` to handle retry logic with proxy fallback, SSE line buffering to prevent chunk-boundary corruption, and graceful shutdown via active reader tracking.
Key changes:
- Add `fetchWithRetry` for automatic direct-to-proxy failover.
- Implement `SSELineBuffer` to ensure reliable parsing of split SSE chunks.
- Add `createStreamBodyLimiter` to enforce payload limits on streaming requests.
- Refactor `ProxyPool` to decouple failure marking from rotation.
- Standardize CORS handling and environment variable configuration.
- Clean up documentation and remove obsolete skill files.
