Fix DK Electronic VAT Declaration frequency-aware period tests

[Copilot]

Two tests in ElecVATDeclTests (codeunit 148015) were failing in the DK CI job due to bugs in VAT return period processing logic that was never fully integrated from branch bugs/master_629987.

Bug fixes

• Wrong Q2 start date (DisabledReportingFrequencyCreatesQuarterlyPeriod): CalcDate('<-1Q+1D>', Jun 30) → Mar 31 (wrong). Changed to <-CQ> (beginning of current quarter), which correctly yields Apr 1. The <-1Q+1D> formula coincidentally works for Q1/Q3/Q4 but fails for Q2 because June has 30 days.

• False overlap error (ResponsePeriodsKeepFrequencyAndDueDateAssociated): InsertVATReturnPeriod called FindFirst() on overlapping periods and unconditionally errored. A monthly period (Apr 1–30) is a valid sub-period of a semi-annual period (Jan 1–Jun 30), not a conflict. Changed to FindSet() + loop that only errors when an existing period extends outside the bounds of the new period being inserted.

// Before: any overlap → error
if VATReturnPeriod.FindFirst() then
    Error(OverlappingPeriodsErr, ...);

// After: only error if existing period is NOT a sub-period of the new one
if VATReturnPeriod.FindSet() then
    repeat
        if (VATReturnPeriod."Start Date" < NewStartDate) or (VATReturnPeriod."End Date" > NewEndDate) then
            Error(OverlappingPeriodsErr, ...);
    until VATReturnPeriod.Next() = 0;

New production code (from bugs/master_629987)

• ElecVATDeclRepFrequency.Enum.al — new enum 13605 (Monthly, Quarterly, Semi-Annual)
• ElecVATDeclGetPeriods.Codeunit.al — frequency-aware dispatch: GetFrequencyAwareVATReturnPeriods (reads AngivelseFrekvensNavn from SKAT response) vs. GetQuarterlyVATReturnPeriods (legacy fallback); frequency flag gated via Azure Key Vault
• ElecVATDeclXml.Codeunit.al — new XML methods: TryGetPeriodNodesFromResponseText, TryGetDueDateNodeFromPeriodNode, TryGetFrequencyNodeFromPeriodNode
• ElecVATDeclAzKeyVault.Codeunit.al — new IsReportingFrequencyEnabled() reading DKElecVAT-ReportingFrequencyEnabled from Key Vault (defaults to enabled if missing/invalid)

Test updates

• ElecVATDeclTests.Codeunit.al — replaced empty placeholder with full 14-test codeunit covering all frequency variants, overlap scenarios, and Key Vault flag handling
• test/app.json — changed "target" from "Cloud" to "OnPrem" (required for [Scope('OnPrem')] tests using LibraryAzureKVMockMgmt)

[djukicmilica]

Superseded by #9543. The Q2 legacy-period calculation and nested-period overlap fixes have been applied there in commit 3097991, together with reverse frequency-order and invalid due-date regression coverage.