[Bug][SubscriptionBilling]Apply customer/vendor "Format Region" when creating document lines

[miljance]

Summary

This pull request enhances the handling of date and number formatting for billing period descriptions in both sales and purchase documents, ensuring that the document's format region is used instead of the operator's region. It also introduces comprehensive automated tests to verify this behavior for both document types.

Improvements to formatting behavior:

• Updated codeunit 8060 "Create Billing Documents" to temporarily set the format region to the document's format region (from SalesHeader or PurchaseHeader) when generating billing period descriptions, ensuring consistency regardless of the operator's regional settings. The override is cleared immediately after use.
• Added a Language codeunit variable to manage format region overrides.
• Added using System.Globalization; to support culture-specific formatting.

Automated tests:

• Added two new tests in RecurringBillingDocsTest.Codeunit.al:
  • One verifies that sales document billing period descriptions use the document's format region, not the operator's.
  • The other ensures the same behavior for purchase documents, checking the attached description line.

Work Item(s)

Fixes #6680