Accept canonical NaN in BFloat16 explicit conversions on WASM

[pavelsavara]

Applies the same fix as #130706 (which handled HalfTests.ExplicitConversion_FromSingle) to the two BFloat16 explicit conversion tests: ExplicitConversion_FromSingle and ExplicitConversion_FromDouble.

The software float/double -> BFloat16 conversions route the value through abs/min/max/add (f32.* / f64.* on WASM). The WebAssembly spec permits (but doesn't require) host engines to canonicalize NaN payloads on those ops, and the V8 engine used on the CI Helix queues does, so the bit-strict NaN cases don't round-trip there. Arch-native targets (and payload-preserving WASM engines) are unaffected.

Previously the NaN cases were skipped entirely via if (PlatformDetection.IsMonoRuntime && PlatformDetection.IsWasm && ...) continue;. That gate only fires on Mono, so once the browser CoreCLR work (#129634) started running these tests on CoreCLR+WASM they began failing on V8 with Assert.Equal() ... Expected: NaN / Actual: NaN (payloads differ). Dropping all NaN coverage on WASM also loses the sign-bit check, which is preserved (carried through the integer ALU).

Instead, this removes the skip and, on WASM only, accepts the canonical result (sign | 0x7FC0 for BFloat16) in place of the exact expected payload:

• a canonicalizing run (V8 today) passes via the canonical value,
• a payload-preserving run (the IEEE 754 / WASM recommended behavior, and what arch-native targets do) still passes via the strict exact-bits check,
• a genuinely wrong payload (e.g. a dropped sign) still fails.

So the NaN cases now actually run on WASM for both Mono and CoreCLR, and the bit-strict AssertEqual stays in effect for all other targets.

Tracked in #103347.

Note

This PR was prepared with the assistance of GitHub Copilot.

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Pull request overview

Updates BFloat16 explicit-conversion tests to be robust on WASM engines that canonicalize NaN payloads (notably V8), while still preserving bit-strict validation on platforms that retain NaN payloads.

Changes:

• Removes the prior WASM+Mono-only skip that dropped NaN test cases from the conversion test data.
• On WASM, allows the conversion result to match either the exact expected NaN payload or the canonical NaN bit pattern (with sign preserved) for float/double → BFloat16 explicit conversions.