refactor: change default delegate to CPU in MediaPipePoseBackend and update tests accordingly

refactor(mediapipe): simplify GPU delegate preference logic and remove unused CUDA visibility check

refactor(mediapipe): enhance MediaPipePoseBackend to prefer tasks on GPU and improve delegate handling

fix(hf-spaces): install mediapipe native libraries

feat(hf): use ZeroGPU for analysis

refactor(mediapipe): improve code readability and add CPU delegate option

feat(pose): add pluggable pose backends