Fix Apps DK (Clean) compile failure by scoping Key Vault mock tests to OnPrem

[Copilot]

Build Apps DK (Clean) / Apps DK (Clean) failed with AL0296 in ElecVATDeclTests.Codeunit.al: Cloud-targeted test procedures invoked Key Vault mock helpers that are OnPrem-scoped.
This PR narrows those tests to OnPrem scope so the Clean Cloud compile no longer attempts illegal calls.

• Root cause

  • ElectronicVATDeclarationDK/test (target: "Cloud") included test methods calling:
    • InitMockAzureKeyvaultSecretProvider
    • AddMockAzureKeyvaultSecretProviderMapping
    • UseAzureKeyvaultSecretProvider
  • These helpers are [Scope('OnPrem')], triggering AL0296 during Apps DK (Clean) compilation.

• Change applied

  • Added [Scope('OnPrem')] to the four reporting-frequency tests that use Key Vault mocks.
  • Added [Scope('OnPrem')] to SetReportingFrequencyKeyVaultFlag local helper used by those tests.

• Illustrative snippet

[Test]
[Scope('OnPrem')]
procedure ReportingFrequencyIsEnabledWhenKeyVaultFlagIsTrue()
begin
    SetReportingFrequencyKeyVaultFlag('true');
    Assert.IsTrue(ElecVATDeclAzKeyVault.IsReportingFrequencyEnabled(), ...);
end;

[Scope('OnPrem')]
local procedure SetReportingFrequencyKeyVaultFlag(FlagValue: Text)
begin
    LibraryAzureKVMockMgmt.InitMockAzureKeyvaultSecretProvider();
    LibraryAzureKVMockMgmt.AddMockAzureKeyvaultSecretProviderMapping('DKElecVAT-ReportingFrequencyEnabled', FlagValue);
    LibraryAzureKVMockMgmt.UseAzureKeyvaultSecretProvider();
end;