Refactor code style and improve documentation

- Standardized string quotes to single quotes across all files.
- Added docstrings to several functions and classes for better clarity.
- Updated mypy configuration in pyproject.toml for enhanced type checking.
- Ignored specific linting rules for test files in ruff configuration.
- Improved error messages in exception handling for better user feedback.
- Cleaned up code formatting and structure for consistency.

tests/test_cli.py

@@ -13,19 +13,19 @@ def test_cli_prints_downloaded_path(
     capsys: pytest.CaptureFixture[str],
     tmp_path: Path,
 ) -> None:
-    expected_path = tmp_path / "video.mp4"
+    expected_path = tmp_path / 'video.mp4'
 
     def fake_download_to_path(**kwargs: object) -> Path:
         del kwargs
         return expected_path
 
-    monkeypatch.setattr(cli, "download_to_path", fake_download_to_path)
+    monkeypatch.setattr(cli, 'download_to_path', fake_download_to_path)
 
     exit_code = cli.main(
         [
-            "https://youtube.com/shorts/example",
-            "cookies.txt",
-            "--output-dir",
+            'https://youtube.com/shorts/example',
+            'cookies.txt',
+            '--output-dir',
             str(tmp_path),
         ]
     )
@@ -39,11 +39,11 @@ def test_cli_prints_downloaded_path(
 def test_cli_exits_with_error(monkeypatch: pytest.MonkeyPatch) -> None:
     def raise_error(**kwargs: object) -> Path:
         del kwargs
-        raise InvalidSessionError("session error")
+        raise InvalidSessionError('session error')
 
-    monkeypatch.setattr(cli, "download_to_path", raise_error)
+    monkeypatch.setattr(cli, 'download_to_path', raise_error)
 
     with pytest.raises(SystemExit) as exc_info:
-        cli.main(["https://youtube.com/shorts/example", "cookies.txt"])
+        cli.main(['https://youtube.com/shorts/example', 'cookies.txt'])
 
     assert exc_info.value.code == 1