fix: handle files without extension in check_file_or_append

[kuishou68]

Summary

Fixes a panic that occurs when check_file_or_append is called with a file path that has no extension (e.g., Makefile, README, or any downloaded binary without a suffix).

Root Cause

The original code unconditionally called .unwrap() on PathBuf::extension(), which returns None for extension-less filenames, causing a runtime panic:

// BEFORE (panics when there is no extension)
let extension = new_path.extension().unwrap().to_string_lossy().to_string();

Fix

Use .map() + unwrap_or_default() for file_stem, and keep extension as an Option<String>. Then branch on whether the extension is present when constructing the new path:

// AFTER (handles missing extension gracefully)
let file_stem = new_path
    .file_stem()
    .map(|s| s.to_string_lossy().to_string())
    .unwrap_or_default();
let extension = new_path
    .extension()
    .map(|e| e.to_string_lossy().to_string());
// ...
new_path = match &extension {
    Some(ext) => parent_dir.join(format!("{new_file_stem}.{ext}")),
    None => parent_dir.join(new_file_stem),
};

This also adds use std::path::Path; to support the Path::new("") fallback for the edge case where parent() returns None.

Closes #1183