Add missing Elec. VAT Decl. Rep. Frequency enum to fix DK Clean build

[Copilot]

PR #9543 introduced frequency-aware VAT return period parsing for the Danish Electronic VAT Declaration app but omitted the enum file it depends on, causing error AL0185: Enum 'Elec. VAT Decl. Rep. Frequency' is missing in the DK Clean CI build.

Changes

• New enum ElecVATDeclRepFrequency.Enum.al (ID 13605, namespace Microsoft.Finance.VAT.Reporting):

  enum 13605 "Elec. VAT Decl. Rep. Frequency"
  {
      value(0; Monthly)   { Caption = 'Monthly'; }
      value(1; Quarterly) { Caption = 'Quarterly'; }
      value(2; "Semi-Annual") { Caption = 'Semi-Annual'; }
  }

  Values correspond to Danish SKAT frequency strings: Måned → Monthly, Kvartal → Quarterly, Halvår → Semi-Annual.

• Includes all three commits from PR Support VAT return reporting frequencies in Denmark #9543 (cherry-picked):

  • Frequency-aware VAT period parsing (GetFrequencyAwareVATReturnPeriods, CalcPeriodEndDate, CalcPeriodStartDate)
  • Feature flag via Azure Key Vault (IsReportingFrequencyEnabled)
  • Overlapping-period guard and legacy quarterly fallback
  • Test coverage for all frequency paths and the feature flag
  • internalsVisibleTo in app.json so the test app can access Access = Internal codeunits