xRechnung: Exporting Email and Data from Header

[miljance]

Summary

Refactored InsertApplicableHeaderTradeAgreement :

• Changed the method signature from accepting CustomerNo and YourReference parameters to accepting a RecordVariant parameter
• Added support for both Sales Invoice Header and Sales Cr.Memo Header using RecordRef and case statement
• Extracted all customer/header fields (Customer No., Name, Address, Post Code, City, Country Code, VAT Reg No., Your Reference, Contact, Email, Phone Number) directly from the header record instead of loading the Customer record separately
• Added email addresses for both Seller (Company) and Buyer (Customer) to the XML output using URIUniversalCommunication elements with schemeID='EM'
• Updated method calls to pass the header record instead of individual parameters

Updated test support:

• Added E-Mail field validation when creating test customers
• Added verification for Seller email in VerifySellerData
• Added verification for Buyer email in VerifyBuyerData methods
• Set "Buyer Reference Mandatory" to true when configuring the E-Document Service

Summary: Corrected the implementation to include email addresses in the ZUGFeRD XML output for both the seller (company) and buyer (customer), making the electronic documents more complete and compliant with ZUGFeRD format requirements.

Work Item(s)

Fixes #29420
AB#614338

[github-actions]

Processing this PR. The branch is now locked 🔒 Please don't push updates unless otherwise agreed.

[JesperSchulz]

This PR has been merged internally. Trying to merge here on GitHub too.