fix(sandbox): await SFTP writes before acknowledging

[drew]

Summary

Ensure the sandbox SFTP handler does not acknowledge a write until Tokio has completed the underlying blocking filesystem operation. This prevents helper shutdown from racing the final buffered write and reporting success while the destination file is still empty.

Related Issue

No issue required: this is an obvious localized correctness fix found while investigating the flaky CI failure in #3550.

Changes

• Flush each tokio::fs::File write before returning the SFTP success response.
• Keep the existing protocol surface and regression test unchanged.

Testing

• mise run pre-commit
• cargo test -p openshell-sandbox --no-fail-fast (228 passed, 1 ignored)
• Existing SFTP round-trip test passed 1,000 consecutive iterations after reproducing the pre-fix failure on iteration 4.
• E2E tests added or updated (not applicable for this localized SFTP write-completion fix)

Checklist

• Follows Conventional Commits.
• Commit is signed off for DCO.
• Documentation reviewed; no user-facing or architecture documentation changes are needed.