Added InvoicePeriod to ItemLine

[stephanstapel]

closes #841

Summary

added invoiceperiod to line items in ubl

Checklist

• [ x] Code follows repo Coding Instructions
• [x ] Public APIs documented (XML)
• [x ] Unit tests added/updated
• Analyzers clean (dotnet build no warnings as errors)
• dotnet format --verify-no-changes passes

Breaking changes?

• [ x] No
• Yes (explain impact & migration)

[Copilot]

Pull request overview

This PR adds support for writing InvoicePeriod (billing period dates) to line items when exporting invoices in UBL format, addressing issue #841.

Key Changes

• Added InvoicePeriod element to line items in UBL output, including StartDate and EndDate child elements based on the TradeLineItem's BillingPeriodStart and BillingPeriodEnd properties

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The condition on this line requires both BillingPeriodStart AND BillingPeriodEnd to be present. However, the document-level InvoicePeriod logic at line 158 uses OR logic (allowing either date), and other format writers like InvoiceDescriptor20Writer.cs also use OR logic. This inconsistency means line items with only a start date or only an end date won't have their InvoicePeriod written in UBL format. Consider changing this to match the pattern: if (tradeLineItem.BillingPeriodStart.HasValue || tradeLineItem.BillingPeriodEnd.HasValue)

Suggested change

	if (tradeLineItem.BillingPeriodStart.HasValue && tradeLineItem.BillingPeriodEnd.HasValue)
	{
	_Writer.WriteStartElement("cac", "InvoicePeriod");
	_Writer.WriteElementString("cbc", "StartDate", _formatDate(tradeLineItem.BillingPeriodStart.Value, false, true));
	_Writer.WriteElementString("cbc", "EndDate", _formatDate(tradeLineItem.BillingPeriodEnd.Value, false, true));
	if (tradeLineItem.BillingPeriodStart.HasValue || tradeLineItem.BillingPeriodEnd.HasValue)
	{
	_Writer.WriteStartElement("cac", "InvoicePeriod");
	if (tradeLineItem.BillingPeriodStart.HasValue)
	{
	_Writer.WriteElementString("cbc", "StartDate", _formatDate(tradeLineItem.BillingPeriodStart.Value, false, true));
	}
	if (tradeLineItem.BillingPeriodEnd.HasValue)
	{
	_Writer.WriteElementString("cbc", "EndDate", _formatDate(tradeLineItem.BillingPeriodEnd.Value, false, true));
	}

[Copilot]

There is no test coverage for the newly added line-level InvoicePeriod functionality in UBL format. While the reader already supports reading InvoicePeriod from line items (InvoiceDescriptor22UblReader.cs lines 545-546), there are no tests verifying that the writer correctly outputs this data. Consider adding a test in XRechnungUBLTests.cs that creates a line item with BillingPeriodStart and BillingPeriodEnd, saves it to UBL format, loads it back, and verifies the dates are preserved.