Why

Fresh app-server thread startup can create a shell snapshot through a temp file and then promote it to the final snapshot path. The previous implementation briefly wrapped the temp path in ShellSnapshot, so after a successful rename its Drop attempted to delete the old temp path and could log a false ENOENT warning.

Fixes #17549.

What changed

• Validate the temp snapshot path directly before promotion.
• Rename the temp path directly to the final snapshot path.
• Keep explicit cleanup of the temp path on validation or finalization failures.