Function expand_path 

pub fn expand_path(raw: &str, ssh_dir: &Path) -> PathBuf

Expand a path that may start with ~ or be relative.

Handles the common SSH config path expansion logic:

• ~/path expands to $HOME/path
• bare ~ expands to $HOME
• Relative paths are resolved against ssh_dir
• Absolute paths are returned as-is

When dirs::home_dir() returns None, ~/ and bare ~ paths are returned unchanged as a PathBuf.