Function merge_paths 

pub fn merge_paths(
    primary: impl AsRef<OsStr>,
    secondary: impl AsRef<OsStr>,
) -> OsString

Merge two PATH strings into a single, de-duplicated PATH.

• Keeps the order of entries from primary.
• Appends only unseen entries from secondary.
• Ignores empty components.
• Returns a platform-correct PATH string (using the OS separator).