Refactor test configurations and enhance testing framework

- Replace TempConfig with DummyConfig across tests for consistency
- Introduce unit tests workflow configuration
- Add pytest configuration for standardized test discovery
- Implement comprehensive tests for config loading and downloader behavior
- Clean up unused temp_config.py and related references

tests/test_downloader_sanitize_and_path.py

@@ -0,0 +1,24 @@
+from pathlib import Path
+
+from ytplaylist.downloader import PlaylistDownloader
+from tests.dummy_config import DummyConfig
+
+
+def test_sanitize_title_and_get_file_path(tmp_path):
+    cfg = DummyConfig()
+    playlist = {"url": None, "save_path": str(tmp_path)}
+    dl = PlaylistDownloader(cfg, playlist, 0)
+
+    # illegal chars should be replaced and trimmed; fallback_id used when title becomes empty
+    title = '  My: <>:"/\\|?*Title  '
+    safe = dl.sanitize_title(title, "ABC123")
+    # ensure no illegal characters remain
+    assert all(c not in safe for c in dl.illegal_chars)
+
+    # empty title should return fallback id
+    assert dl.sanitize_title("   ", "FALLBACK") == "FALLBACK"
+
+    # get_file_path uses save_path and zero-padded index
+    path = dl.get_file_path(5, "SongName")
+    assert isinstance(path, Path)
+    assert path.name.startswith("005 - SongName")