Add test hook to PopKeyAttestor for MSI v2 attestation tests

[Copilot]

Changes proposed in this request

Unit tests calling .WithAttestationSupport() fail because PopKeyAttestor.AttestCredentialGuardAsync() loads AttestationClientLib.dll, which throws TaskCanceledException in test environments and makes network calls that bypass MockHttpManager.

Solution: Test hook pattern to inject mock attestation provider.

Core changes:

• PopKeyAttestor.cs: Added s_testAttestationProvider static field checked before native DLL loading
• TestAttestationProviders.cs: Replaced unused CreateFakeProvider() with CreateFakeAttestationResultProvider() returning AttestationResult
• ImdsV2Tests.cs: Inject mock in TestInitialize, reset in TestCleanup

// In PopKeyAttestor.AttestCredentialGuardAsync()
if (s_testAttestationProvider != null)
{
    return s_testAttestationProvider(endpoint, keyHandle, clientId, cancellationToken);
}
// Otherwise proceed with native DLL...

All 27+ existing test calls to .WithAttestationSupport() now bypass native DLL without modification.

Testing

Test infrastructure updated. Existing unit tests in ImdsV2Tests now use the mock provider automatically. No new tests added—fix enables existing tests to run.

Performance impact

None. Production code path unchanged. Test-only hook accessed via InternalsVisibleTo.

Documentation

• All relevant documentation is updated.

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[bgavrilMS]

Too much test code in the product code for 1 trivial test.