Run a follow-up action on a completed Udio job

Runs a follow-up action on a completed udio job, referenced by its job ID. Both actions select one of the parent's songs with index (1–2) or trackId (from the parent's result.tracks[].id). extend adds up to 32 seconds (output_length) to the song, placed before or after it (placement). One extension per call — to build a longer track, extend the result again. remix generates the song again: remix_strength (0.1–1.0, default 0.5) sets how far from the original it lands, and crop_range limits it to part of the track. Anything left out is inherited from the source song; the optional generation fields override it. Both return a fresh pair of Udio songs that can be acted on again. Extending is not available for songs generated with a udio130-* model or longer than 15 minutes. Returns 409 when the action is not available for the parent job.