Closes#588. Contributing-artist tagging worked for some tracks but
silently dropped them for others — most reproducibly when the album
had been fetched before the per-track post-process ran.
Trace: get_track_details cache check used `track_position in cached`
as the "full payload" sentinel. Both `/track/<id>` AND
`/album/<id>/tracks` set track_position. Only `/track/<id>` sets the
`contributors` array. When album-tracks data hit the cache first,
get_track_details returned the partial record →
_build_enhanced_track found no contributors → metadata-source
contributors-upgrade silently fell back to single-artist.
Reporter's case (Andrea Botez - Sacrifice): the album fetch logged
"Retrieved 4 tracks for album 673558211" before the post-process,
which cached all 4 tracks as partial records. The contributors-
upgrade then hit the partial cache and the upgrade log line never
fired because len(upgraded) was never > 1.
Lifted cache-validity to a pure helper `_is_full_track_payload` that
requires BOTH `track_position` AND `contributors` key presence. Empty
list `[]` is valid — single-artist tracks fetched via `/track/<id>`
carry it explicitly. Partial cache hits fall through to a fresh
`/track/<id>` fetch, which writes the full payload back to cache.
11 boundary tests pin every shape: full payload, single-artist with
empty contributors list, partial album-tracks shape, search-result
shape, none/non-dict, and the cache-hit/cache-miss/api-failure paths
on get_track_details (including the exact reporter-scenario
regression).
Full suite: 3021 passed.
Broque Thomas