[MLAS] Add fp16 extreme-value coverage for ARM64 HGEMM NEON

[kjg0724 (kjg0724)]

Summary

test_hgemm_neon.cpp's four arithmetic test classes (MlasNeonHGemmTransposedBTest, MlasNeonHGemmBTest, MlasNeonHGemmPackedBTest, MlasNeonHGemmTest) only ever fill operands via a uniform_real_distribution restricted to [-1,1] or [-0.25,0.25]. Even the largest shapes (K up to 513) never come close to fp16 overflow (max ~65504) since the accumulated magnitude stays well under that.

FloatEqual (duplicated identically across all four classes) also mishandles Inf/NaN:

return std::abs(f0 - f1) <= std::abs(f1 * rtol) + atol;

Inf == Inf computes abs(Inf-Inf) = NaN, and NaN <= Inf is false, so two matching infinities fail the comparison. Conversely, a finite kernel output against an Inf reference computes abs(finite-Inf) = Inf <= Inf, which is true -- a dropped overflow passes silently. Extracted the function into a single free function with explicit Inf/NaN handling; the existing finite-value branch is untouched.

Added TestHGemmFixed<> to each class, filling operands with fixed extreme fp16 values instead of the random distribution: near max magnitude, alpha/beta scaling that pushes into overflow, and subnormal-range products. Values were chosen to land in ranges where the kernel's native fp16 accumulation and the reference's fp32-then-round accumulation agree. Large-magnitude operands combined with large K diverge past the existing rtol=0.02/atol=0.055 tolerance from fp16's accumulation error alone (confirmed by measurement, e.g. a=10,b=1,K=513 gives 4928 vs a 5128 reference), so those combinations are deliberately avoided rather than loosening the tolerance.

MlasNeonHGemmPackBTest is left untouched: HPackB_TransposedB_Kernel/HPackB_B_Kernel only move data, and Check<N,K> already compares raw bit patterns (ASSERT_EQ(packed[...].val, ref[...].val)), so FloatEqual's defect doesn't apply there.

Test-only change; no file under onnxruntime/core/mlas/lib/ is touched.

Testing

test_hgemm_neon.cpp only builds under MLAS_F16VEC_INTRINSICS_SUPPORTED && MLAS_TARGET_ARM64; under the default build configuration that excludes Apple targets (mlas.h disables fp16 vector intrinsics under __APPLE__), so verification was done on a non-Apple ARM64 host.

Neoverse-N1 (Oracle Cloud A1, Ubuntu):

• *NeonHGemm*: 5/5 passed (all five suites, including the new extreme-value cases)
• Full onnxruntime_mlas_test suite: 36020/36020 passed, no regressions

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.