Harden head-size-256 paged XQA coverage (#32262)
Addressing open comments from:
https://github.com/microsoft/onnxruntime/pull/32229#pullrequestreview-5004757770
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: a8d618bb-0feb-4d05-a918-93b6accde1b1
