Fix VAT Period validation error in PEPPOL tests

[Copilot]

Test ExportXml_PEPPOL_BIS3_SalesInvoice_CheckTaxTotalWhenUnitPriceZero failed when Czech localization validated VAT periods for test dates that don't exist in the test environment.

Changes

• Disable VAT Reporting Date Usage in GL Setup during test initialization to prevent localization-specific VAT period validation

GLSetup.GetRecordOnce();
GLSetup."VAT Reporting Date Usage" := GLSetup."VAT Reporting Date Usage"::Disabled;
GLSetup.Modify(false);

This aligns with the existing pattern in PEPPOL30ManagementTests.Codeunit.al.

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