feat: add OpenComputer sandbox provider

[sidpalas]

Summary

• add an opencomputer sandbox backend to the control-plane, including provider selection, API client wiring, lifecycle handling, preview URL support, and focused unit coverage
• add a sandbox-authenticated fatal error reporting path so runtime startup failures surface in the session instead of hanging in connecting
• document and script the OpenComputer runtime snapshot build flow used for local validation, including the runtime bootstrap expected by the provider

Validation

• npm test -w @open-inspect/control-plane -- src/sandbox/providers/opencomputer-provider.test.ts src/sandbox/provider-name.test.ts src/session/http/routes.test.ts
• npm run typecheck -w @open-inspect/control-plane
• validated the OpenComputer lifecycle API directly against the hosted service during development