Add unit tests for XRechnung payment terms formatting

[jswzr]

This PR adds the unit tests discussed in Issue #721, specifically covering the following scenarios:

• Ensures only one SpecifiedTradePaymentTerms element is present in XRechnung documents, as required by the specification.

• Verifies that multiple payment conditions (e.g. Skonto terms) are correctly structured within the Description element using the required line-based format.

• Confirms that each structured payment condition ends with a proper line break (\n), ensuring compatibility with XRechnung validators.

These tests help ensure conformance with the XRechnung CIUS and improve validation reliability for structured payment term formatting.

[stephanstapel]

Hi @jswzr ,

thanks for your pull request. Can you review

ZUGFeRD-csharp/ZUGFeRD.Test/ZUGFeRD22Tests.cs

Line 2880 in a9987a7

public void TestPaymentTermsMultiCardinalityXRechnungStructured()

do you think this is still necessary?

Concerning newline, please use this constant:

ZUGFeRD-csharp/ZUGFeRD/XmlConstants.cs

Line 32 in a9987a7

public const string XmlNewLine = "
";

Finally, could you add to your proposed tests reading of the raw file and checking if tab and newline (as mentioned above) is written correctly?

[jswzr]

Thanks again for the feedback!

To me, it absolutely makes sense to test multiple payment terms – they’re all written into a single field with a custom structure.

Using XmlConstants.XmlNewLine definitely makes sense – I’ll update the tests accordingly.

Regarding the last point:
Currently, GetTradePaymentTerms() returns the structured lines including line breaks via the Description property.
Would you prefer I manually open and read the generated file (e.g., using XmlDocument) and directly check the PaymentTerms section to ensure the correct use of newlines in the raw XML?

Just want to be sure I understand your expectations for this test.

[stephanstapel]

Hi again,
great, thanks!
concerning the last point: in addition to reading the xml as xml (using either the library o XmlDocument), it would be great to convert the stream into a string and see if indention and line breaks are as they are supposed to be. I'd like to make sure that future changes don't break the required textual representation.

Like this;

StreamReader reader = new StreamReader( stream );
string content = reader.ReadToEnd();

you can find a similar test here:

ZUGFeRD-csharp/ZUGFeRD.Test/XRechnungUBLTests.cs

Line 479 in a9987a7

public void TestSingleSkontoForCorrectIndention()