Function handle_single_input_argument 

pub(crate) fn handle_single_input_argument(arg: &str) -> Result<()>

Handles a single input argument, either a GitHub PR URL or a branch name, and switches to the corresponding branch.

Behaviour:

• If arg parses as a GitHub PR URL, authenticate then derive the branch name and switch to it.
• Otherwise, use arg as the branch name and switch to it.

Errors

• GitHub authentication via [ytil_gh::log_into_github] fails (if URL).
• Pull request branch name derivation via [ytil_gh::get_branch_name_from_url] fails (if URL).
• Branch switching via [ytil_git::branch::switch] fails.