fix(sandbox): replace forEach(async) with Promise.all to prevent race condition on multi-file writes

[Prasad48432]

Summary

Fixes #229.

Resolves a race condition in the sandbox API route where sbx.runCode() could
be invoked before all sandbox file writes had completed, causing code execution
against a partially-written filesystem for multi-file fragments.

---

Root Cause

In app/api/sandbox/route.ts, the multi-file write path used:

fragment.code.forEach(async (file) => {
  await sbx.files.write(file.file_path, file.file_content)
  console.log(`Copied file to ${file.file_path} in ${sbx.sandboxId}`)
})

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