Add ARM64 support to GetArchitectureForSource methods

[Copilot]

The GetArchitectureForSource methods in AssemblyMetadataProvider and AssemblyLoadWorker were missing support for ARM64 native assemblies. When VsTest encountered ARM64 binaries, it would fail to detect the correct architecture and fall back to either the user-specified default or the machine's architecture, resulting in incorrect testhost selection.

This change adds the missing ARM64 constant and switch case to both methods:

Changes made:

1. Added IMAGE_FILE_MACHINE_ARM64 = 0xAA64 constant (matching MSBuild's definition)
2. Added ARM64 case to the machine type switch statements to return Architecture.ARM64

Before:

switch (machine)
{
    case imageFileMachineI386:
        archType = Architecture.X86;
        break;
    case imageFileMachineAmd64:
    case imageFileMachineIa64:
        archType = Architecture.X64;
        break;
    case imageFileMachineArm:
    case imageFileMachineThumb:
    case imageFileMachineArmnt:
        archType = Architecture.ARM;
        break;
    // ARM64 missing - would fall through to default
}

After:

switch (machine)
{
    case imageFileMachineI386:
        archType = Architecture.X86;
        break;
    case imageFileMachineAmd64:
    case imageFileMachineIa64:
        archType = Architecture.X64;
        break;
    case imageFileMachineArm:
    case imageFileMachineThumb:
    case imageFileMachineArmnt:
        archType = Architecture.ARM;
        break;
    case imageFileMachineArm64:
        archType = Architecture.ARM64;
        break;
}

This ensures VsTest can now correctly identify ARM64 native assemblies and select the appropriate ARM64 testhost, improving test execution on ARM64 platforms.

Fixes #15275.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• 37cvsblobprodcus359.vsblob.vsassets.io
  • Triggering command: dotnet build src/Microsoft.TestPlatform.ObjectModel/Microsoft.TestPlatform.ObjectModel.csproj (dns block)
• 4m6vsblobprodcus384.vsblob.vsassets.io
  • Triggering command: dotnet build src/Microsoft.TestPlatform.ObjectModel/Microsoft.TestPlatform.ObjectModel.csproj (dns block)
• 5dkvsblobprodcus355.vsblob.vsassets.io
  • Triggering command: dotnet build src/Microsoft.TestPlatform.ObjectModel/Microsoft.TestPlatform.ObjectModel.csproj (dns block)
• c50vsblobprodcus330.vsblob.vsassets.io
  • Triggering command: dotnet build src/Microsoft.TestPlatform.ObjectModel/Microsoft.TestPlatform.ObjectModel.csproj (dns block)
• fdpvsblobprodcus345.vsblob.vsassets.io
  • Triggering command: dotnet build src/Microsoft.TestPlatform.ObjectModel/Microsoft.TestPlatform.ObjectModel.csproj (dns block)
• l49vsblobprodcus358.vsblob.vsassets.io
  • Triggering command: dotnet build src/Microsoft.TestPlatform.ObjectModel/Microsoft.TestPlatform.ObjectModel.csproj (dns block)
• pkvvsblobprodcus321.vsblob.vsassets.io
  • Triggering command: dotnet build src/Microsoft.TestPlatform.ObjectModel/Microsoft.TestPlatform.ObjectModel.csproj (dns block)
• yttvsblobprodcus357.vsblob.vsassets.io
  • Triggering command: dotnet build src/Microsoft.TestPlatform.ObjectModel/Microsoft.TestPlatform.ObjectModel.csproj (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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