fix: Handle TD1 embedded document check digits in optional1 for non-compliant documents like PRT and BEL IDs

[uwolfer]

Some TD1 issuers embed the document number check digit inside optional1 when the check-digit slot is <. This update validates the embedded digit against the concatenated document number + optional prefix before falling back to the ICAO-style separator variant.

Notes:

• No official public specification found for embedded document‑number check digits in optional1.
• The anonymized test case is derived from a real example that cannot be shared.

This is related to #66.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.35%. Comparing base (f716198) to head (f89e4b7).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
+ Coverage   88.66%   89.35%   +0.69%     
==========================================
  Files          48       48              
  Lines         450      451       +1     
  Branches      123      122       -1     
==========================================
+ Hits          399      403       +4     
+ Misses         51       48       -3     

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[stropitek]

The title of the PR is a bit misleading here. We are already handling check digits which are in the optional field, but this is about how the check digit of the document number is calculated, specifically how the input of the check digit calculation is constructed.

I found a PRADO example which indeed has other logic for computing the check digit: https://www.consilium.europa.eu/prado/en/BEL-BO-03003/index.html

Can you use this one instead of your custom built one for the unit test?

The logic you added in parseDocumentNumberCheckDigit SGTM. Could you add a comment there on the part you added that it handles older non-compliant documents like PRT and BEL IDs. A reference to https://www.icao.int/sites/default/files/publications/DocSeries/9303_p11_cons_en.pdf page 88 to support that would also be useful for future reference.