[webgpu] Simplify the signature of CanApplyFlashAttention (#26926)
This pull request simplifies the logic for handling present key/value
tensors in the WebGPU Flash Attention implementation. The main change is
that the responsibility for creating internal present key/value tensors
is moved from the caller to the `ApplyFlashAttention` function itself.
This reduces code duplication and makes the API easier to use.
Additionally, the `CanApplyFlashAttention` function is simplified to
remove unnecessary checks for present key/value tensors.