Add sandbox pooling for performance

## Description
Implement sandbox pooling to reuse sandbox instances for better performance.

Currently, each execution creates a new sandbox and destroys it afterward. For high-throughput scenarios, maintaining a pool of warm sandboxes would significantly reduce latency.

## Requirements
- [ ] Implement pool manager with configurable size
- [ ] Pre-warm sandboxes on startup
- [ ] Reset sandbox state between uses
- [ ] Handle pool exhaustion (queue or create new)
- [ ] Cleanup idle sandboxes after timeout
- [ ] Per-provider pool configuration

## Considerations
- Memory usage vs latency tradeoff
- Sandbox isolation between uses
- Different pools per language/runtime