Function plan_rename 

pub async fn plan_rename(
    ctx: &RefactoringContext,
    def_rel_path: &str,
    old_name: &str,
    new_name: &str,
    force: bool,
) -> Result<RefactoringPlan, String>

Build a rename plan without touching the filesystem.

Returns a RefactoringPlan containing all edits needed, or an error if the target can’t be resolved or conflicts are detected.