Disable TpaLoadFailureTest cases on mobile platforms

[Copilot]

TpaLoadFailureTest.NotFound_ExceptionContainsAssemblyPath and TpaLoadFailureTest.Corrupt_ExceptionContainsPathAndHResult fail on iossimulator-arm64 (AllSubsets_CoreCLR) because the expected FileNotFoundException is not thrown — TPA bind behavior differs on mobile CoreCLR.

Add PlatformDetection.IsNotMobile to the ConditionalFact conditions on both tests so they are skipped on mobile targets.

[ConditionalFact(typeof(PlatformDetection),
    nameof(PlatformDetection.IsCoreCLR),
    nameof(PlatformDetection.HasAssemblyFiles),
    nameof(PlatformDetection.IsNotMobile))]
public void NotFound_ExceptionContainsAssemblyPath() { ... }

The third test in the file (SharingViolation_ExceptionContainsPathAndHResult) is already Windows-only and unaffected.