Tests: import librosa only when available in the Qwen audio tests
Qwen2.5-Omni, Qwen2-Audio and Qwen3-Omni-MoE imported librosa at module level, so without it the whole file failed
to collect. Only the integration tests load audio with it: they now require it, and the rest run.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>