Phase 2. Add create_playlist(title, tracks, public) to the LB client: POST /playlist/create
for the MBID, then add tracks in batches of 100 (MAX_RECORDINGS_PER_ADD) via the item/add
endpoint so 1k-track playlists don't hit a single-request cap. Returns a result dict
{success, playlist_mbid, playlist_url, added, requested, error} and never raises — partial
add failures are reported honestly (playlist created, added count accurate). Extends the
existing token-auth client; additive. 4 mocked-network tests (batching, auth, failure).
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