Update StableAudioStrategy and SamplePackHandler for improved audio generation and preview URL handling

Commit notes

- Renamed StableAudioStrategy to reflect "Stable Audio Open" in comments and method names. - Adjusted falRequest structure to include new fields for audio generation parameters. - Updated audio generation logic to use the new API endpoint and modified response handling for audio file retrieval. - Simplified preview URL generation in SamplePackHandler to skip token generation for user-owned packs, enhancing performance and loading times.