Recover the BIC when UBL nests it inside the financial institution

[rilla]

Context

UBL can carry the payee's BIC in two places: on cac:FinancialInstitutionBranch/cbc:ID (BT-86), or one level deeper under cac:FinancialInstitution/cbc:ID. The parser only read the first, so a document using the nested form lost the BIC entirely.

test/data/parse/peppol/nbio-stuck-ubl.xml is one of those documents — its GKCCBEBB never reached the parsed invoice, as the golden output shows. The wire model already declared FinancialInstitution for this element, but nothing read it.

Changes

• Fall back to the nested FinancialInstitution/ID when the branch carries no ID of its own.
• Assert the recovered BIC on the Peppol fixture that exercises the nested form.
• Use cbc.Code's IsEmpty in newCreditTransferAccount, now that those fields are codes.

Outbound is unchanged: the BIC still goes to BT-86. The fallback only fires where the parser previously produced nothing, so no existing document parses differently.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.46%. Comparing base (58fb61a) to head (73604ce).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #124      +/-   ##
==========================================
+ Coverage   81.40%   81.46%   +0.05%     
==========================================
  Files          28       28              
  Lines        2167     2174       +7     
==========================================
+ Hits         1764     1771       +7     
  Misses        265      265              
  Partials      138      138              

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[rilla]

Closing this. It came out of reading around the v0.504.0 payment change rather than from a reported problem, and the case for it rests on the nested cac:FinancialInstitution form being something we should accept — which I never confirmed against EN 16931. If the form shows up in real traffic, the branch keeps the fix and its test.