Add sb.terminate({ wait: true }) parameter to JS

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PR Summary

Low Risk
Small, backwards-compatible API extension in the JS client with test updates; risk is limited to subtle behavioral changes around termination/detach ordering when wait is used.

Overview
Adds SandboxTerminateParams and overloads Sandbox.terminate() to accept terminate({ wait: true }), which waits for completion and returns the sandbox exit code (otherwise preserves the void behavior).

Exports the new type from src/index.ts and updates sandbox/proxy tests to assert terminate({ wait: true }) === 137 instead of terminating and then re-attaching to call wait() separately.

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