Improve PagedAttention dispatch diagnostics (#32099)
### Summary
Extend the existing CUDA attention debug output with
PagedAttention-specific dispatch details.
When `ORT_ENABLE_ATTENTION_KERNEL_DEBUG_INFO=1`, `PagedAttention` now
reports:
- the selected backend, including XQA versus portable paged decode;
- the number of decode splits;
- the GQA group size;
- the effective KV-length bound after applying the local window;
This makes it easier to verify that an intended optimized path is
selected instead of silently falling back. The diagnostics are opt-in
and do not change kernel selection or execution when disabled.
