Fix AL0296 errors in DK Clean build: add VAT return reporting frequency support

[Copilot]

The "Build Apps DK (Clean)" CI job was failing with 8 AL0296 errors because the Electronic VAT Declaration for Denmark Tests app declared "target": "Cloud" while its new tests called [Scope('OnPrem')] Azure Key Vault mock methods.

Root cause

test/app.json had "target": "Cloud", blocking use of InitMockAzureKeyvaultSecretProvider, UseAzureKeyvaultSecretProvider, and AddMockAzureKeyvaultSecretProviderMapping — all [Scope('OnPrem')] — even from test procedures individually marked [Scope('OnPrem')].

Changes

• test/app.json — "target": "Cloud" → "target": "OnPrem" (the fix)
• app/app.json — added internalsVisibleTo entry for the test app
• ElecVATDeclRepFrequency.Enum.al (new) — Monthly, Quarterly, Semi-Annual enum values
• ElecVATDeclAzKeyVault.Codeunit.al — added IsReportingFrequencyEnabled() reading DKElecVAT-ReportingFrequencyEnabled from AKV; defaults to true when secret is missing or unparseable
• ElecVATDeclGetPeriods.Codeunit.al — refactored to dispatch between frequency-aware (GetFrequencyAwareVATReturnPeriods) and legacy quarterly-only (GetQuarterlyVATReturnPeriods) paths; added CalcPeriodEndDate/CalcPeriodStartDate/GetReportingFrequency; overlap detection on insert
• ElecVATDeclXml.Codeunit.al — added TryGetPeriodNodesFromResponseText, TryGetDueDateNodeFromPeriodNode, TryGetFrequencyNodeFromPeriodNode for period-level XML traversal
• ElecVATDeclTests.Codeunit.al — replaced placeholder with tests covering period date math, frequency-aware parsing, duplicate prevention, overlap detection, and AKV flag evaluation