fix(modal): modal sandboxes fail to upload files (#657)

### Summary

Modal v0.8.0 released the following breaking change:
```
Breaking: Removed the deprecated low-level file-handle API: Sandbox.Open / SandboxFile (Go), and sandbox.open / SandboxFile (JS). Use the Sandbox Filesystem API instead: sandbox.Filesystem() (Go) / sandbox.filesystem (JS).
```
Ref:
https://github.com/modal-labs/modal-client/blob/main/CHANGELOG_GO_JS.md

This PR updates our implementation to use the sandbox filesystem api

### Tests

Updated `sandbox.test.ts` mocks to model `sandbox.filesystem`