Fire a fire-and-forget HEAD request early in startup to warm the TCP+TLS connection. Subsequent SDK calls share an undici dispatcher with preconnect, reusing the warmed connection to save 100-200ms on the first request.
Skip conditions:
- NODE_EXTRA_CA_CERTS set (enterprise TLS inspection)
- Sandbox mode (process-restart context)
- Non-default baseUrl (mTLS / private deployment)
- Non-Node runtimes (Bun)
Disable via QWEN_CODE_DISABLE_PRECONNECT=1.
Closes#3223
