diffusers added `callback_on_step_end` per pipeline rather than across the
board, and audio_gen passed it unconditionally. StableAudioPipeline does not
take it, so every request raised

    StableAudioPipeline.__call__() got an unexpected keyword argument
    'callback_on_step_end'

AFTER the model had finished loading -- one unsupported keyword cost the whole
load and read like a broken model rather than a wrong argument.

Asked of the signature, not kept as a list of pipeline names: such a list is
wrong again at the next diffusers release. The pre-0.25 `callback` spelling is
adapted so those pipelines keep their progress, and a pipeline that supports
neither says so, because silently losing progress reporting and mid-run
cancellation should not be invisible.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014T3rZozi1Ehk96NG1WesgC