Change Summary

Summary

This change hardens on-demand snapshot export behavior for issue #2872 using Typesense-side safeguards only.

Fixes

• Snapshot requests are now guarded earlier, so overlapping snapshots are rejected before they start.
• Timed snapshots and on-demand snapshots now use the same in-progress guard.
• The snapshot request path handoff was cleaned up so the export path is carried through the closure instead of shared mutable state.
• External exports now write to a temporary directory first.
• External exports move into place only after the copy succeeds.
• Failed exports should no longer partially overwrite a previous good snapshot.
• The braft snapshot-reader patch was removed; external export still copies from the raft snapshot directory.

Result

• Concurrent snapshot requests now return 409 Another snapshot is in progress instead of running in parallel.
• Snapshot export is safer for the destination path under heavy write load.
• Previous good external snapshots are protected from partial overwrite on export failure.

PR Checklist

• I have read and signed the Contributor License Agreement.