WebGPU: Optimize GraphCacheManager by removing pending_buffers (#27453)
This pull request refactors the buffer management logic in the
`GraphCacheManager` class to simplify buffer handling and improve
initialization. The main changes focus on removing the use of a pending
buffer queue and ensuring buckets are properly initialized.
Benefits:
✅ Simpler, more efficient code
✅ Immediate buffer reuse - no waiting for OnRefresh()
✅ Less memory overhead