Add modal sandbox support

[waltertang27]

I was able to successfully run the test:

npx vitest run 2>&1

 RUN  v3.2.4 /home/ec2-user/sandbox-agent/examples/modal

stdout | tests/modal.test.ts > modal example > starts sandbox-agent and responds to /v1/health
Creating Modal sandbox!

stdout | tests/modal.test.ts > modal example > starts sandbox-agent and responds to /v1/health
Sandbox created: sb-voOs84Vv30H9czg9Y2fvuI
Starting server...

stdout | tests/modal.test.ts > modal example > starts sandbox-agent and responds to /v1/health
Waiting for server...

 ✓ tests/modal.test.ts (1 test) 8842ms
   ✓ modal example > starts sandbox-agent and responds to /v1/health  8840ms

 Test Files  1 passed (1)
      Tests  1 passed (1)
   Start at  17:43:50
   Duration  9.33s (transform 103ms, setup 0ms, collect 281ms, tests 8.84s, environment 0ms, prepare 68ms)