Introduce ConnTracker (atomic.Bool + WaitGroup) to track in-flight proxy
connections per sandbox. Before pausing a VM, the manager drains active
connections with a 2s grace period, preventing Go runtime corruption
inside the guest caused by stale TCP state surviving Firecracker
snapshot/restore.
Also add:
- AcquireProxyConn on Manager for atomic lookup + connection tracking
- Proxy cache (120s TTL) on CP SandboxProxyWrapper with single-query
DB lookup (GetSandboxProxyTarget) to avoid two round-trips
- Reset() on ConnTracker to re-enable connections if pause fails
Add /proxy/{sandbox_id}/{port}/* handler that reverse-proxies HTTP
requests to services running inside sandbox VMs. The sandbox's host IP
(10.11.0.{idx}) is used as the upstream target.
Includes port validation (1-65535) and shared HTTP transport for
connection pooling. Supports WebSocket upgrades for protocols like
Jupyter's streaming API.
This is an intermediate state — needs further work for the full code
interpreter feature.
