Function apply_drop_in_shell_script 

pub fn apply_drop_in_shell_script(choice: SudoTimestampChoice) -> String

What: Build a POSIX shell script that validates and installs the drop-in with sudo.

Inputs:

• choice: Selected cache duration.

Output:

• A script suitable for spawn_shell_commands_in_terminal: writes temp file, visudo -cf, install -m 0440.

Details:

• Ends with read so the user can see success/failure in an external terminal.
• Uses SUDOERS_DROP_IN_PATH as the destination path.