[MLAS][KleidiAI] Apply fp16 split independent fixes

[Laan33]

Description

This PR splits out a small set of independent fixes and test-scope updates from the larger CPU fp16 enablement work.

Changes:

• Adds KleidiAI Conv2D regression coverage for LHS indirection cache behaviour when the per-thread padding buffer grows.
• Runs that regression in both short and long Conv2D test paths when KleidiAI convolution is available.
• Disables WebGPU for the fp16 Attention test case that uses mask_index, which WebGPU Attention does not support.
• Renames the dynamic QGEMM batch-count parameter from BatchSize to BatchN for consistency with MLAS batch naming.

Motivation and Context

These changes are independent from the wider fp16 MLAS/API and CPU EP enablement work in #28487.

The Conv2D regression exercises the small-CI -> large-CI -> small-CI sequence that can expose stale cached LHS pointer tables after the KleidiAI padding buffer grows. The Attention test exclusion keeps the test targeted to providers that support the tested mask_index input combination.

Note: This PR is the first offshoot from #28487.