test(integration): use flat artist_handle field on track response (#1360)
the GET /tracks/{id} response shape is flat (`artist`, `artist_handle`,
`artist_did` as siblings of the track fields), not nested. the
integration test was reading `track['artist']['handle']`, which crashed
the print statement *after* the upload had already succeeded — so the
test reported a fail despite the server-side path being healthy.
caught while smoke-testing the worker process split (#1359) on staging.
Co-authored-by: Claude Opus 4 (1M context) <noreply@anthropic.com>
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