Fix Flash and Lean attention split heuristics (#29554)
### Description
Fixes split heuristic edge cases in Flash Attention and Lean Attention
when the SM count is reported as zero or when there are no key tiles.
The change clamps the SM count used by the heuristics, prevents
divide-by-zero paths, and returns stable buffer sizing for empty tile
cases.
### Changes
- Guard Flash Attention split heuristic against zero SM count and
degenerate split counts.
- Guard Lean Attention split/buffer sizing against zero SM count, zero
key tiles, and single-tile division cases.
- Add CUDA provider regression tests for zero SM count and zero key-tile
inputs.
### Testing
- Added `attention_split_heuristic_test.cc` coverage for Flash Attention
and Lean Attention split heuristics.
Fixes #29550
