v0.5.2 - YouTube Source Reliability Improvements

Fixed

• Falsey list bug in youtube.py: Passing player_clients=[] no longer gets overridden with defaults. Uses explicit is not None check to distinguish between None (use defaults) and [] (use yt-dlp defaults).
• Playlist expansion failure handling: Failed playlist/channel expansions now raise DiscoveryError immediately instead of returning the original URL, which would cause confusing "file not found" errors downstream.

Added

• New DiscoveryError exception: Structured exception for source discovery failures with optional URL context, following existing exception hierarchy.
• Robust URL detection: _is_youtube_collection() helper uses urllib.parse for proper URL parsing instead of brittle substring matching.
• Comprehensive test coverage: 20 new tests in tests/test_source_discovery.py covering DiscoveryError, URL detection, playlist expansion, and player_clients handling.