fix(deezer): harden metadata parsing against missing API fields

[berettavexee]

Problem

Two KeyError / IndexError cases in the Deezer metadata parsers cause downloads to fail silently on certain content:

1. resp["contributors"] — KeyError

AlbumMetadata.from_deezer() and TrackMetadata.from_deezer() access resp["contributors"] directly. This key is absent for some tracks and albums (singles, compilations, or responses from certain regional API endpoints), raising a KeyError that aborts the download.

2. resp["tracks"][-1] — IndexError

AlbumMetadata.from_deezer() uses resp["tracks"][-1]["disk_number"] to determine disctotal. On geo-restricted albums the tracks list can be empty, causing an IndexError.

Fix

# album.py / track.py — guard contributors
contributors = resp.get("contributors", [])
albumartist = ", ".join(
    c["name"] for c in contributors if c["type"] == "artist"
) or typed(safe_get(resp, "artist", "name"), str)

# album.py — guard empty track list
disctotal = typed(resp["tracks"][-1]["disk_number"], int) if resp["tracks"] else 1

Test plan

• Album with contributors in response → artists joined correctly
• Album without contributors key → falls back to resp["artist"]["name"]
• Geo-restricted album with empty tracks list → disctotal defaults to 1, no crash
• Normal album download unaffected

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