fix(engine): correct mock call index in multi-track audioMixer test (#1144)

processCompositionAudio prepares all tracks in parallel (Promise.all),
so for N tracks the mix call lands at index N, not index 1. The 3-track
test was reading calls[1] (the second prepare call) instead of calls[3]
(the mix call), causing indexOf("-filter_complex") to return -1 and the
subsequent assertions to read the wrong args.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>