Fix validation erros and parsing

[alvarolivie]

This was originally tackled due to an issue with CII not having a field that specified an ammount and gobl parsed that as an empty amount which is 0. This has led me down a rabbit hole where we there where a few issues parsing. I'll describe these in more detail bellow:

• Advances, when paid in full, are not set so we were parsing it as 0. In these cases the real amount should be the whole invoice.
• Advances need a description that CII does not provide
• We were checking for the pointer to an ID but not the actual value.
• When we have a base and an amount we need to create a fake percentage from the two as base only works with percent.
• Legal Organization was not being parsed. Can contain identities.

[Copilot]

Pull Request Overview

This PR fixes several validation errors and parsing issues in the CII (Cross Industry Invoice) parser, particularly around handling advances, allowances/charges, and party identities. The changes ensure that empty amounts are properly handled, missing descriptions are provided, and percentage calculations are correctly derived when only base amounts are available.

• Fixes advance payment parsing to handle full payments and provide required descriptions
• Adds validation check to test suite to catch parsing errors earlier
• Implements percentage calculation from base amounts when percentage is not explicitly provided
• Adds support for parsing Legal Organization identities from party data

Reviewed Changes

Copilot reviewed 13 out of 28 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
settlement_parse.go	Adds descriptions for advances and improves payment term parsing logic
allowance_charge_parse.go	Implements percentage calculation from base amounts for charges and discounts
party_parse.go	Adds support for parsing Legal Organization identities
lines_parse.go	Adds null checks for product ID fields before assignment
examples_test.go	Adds validation check to test suite
party_parse_test.go	Updates test expectations for new identity parsing
test/data/convert/xrechnung/invoice-partial-payment.json	New test data file for partial payment scenarios
test/data/parse/*.xml	Test data updates with consistent tax registration IDs

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 56.92308% with 28 lines in your changes missing coverage. Please review.

Project coverage is 71.54%. Comparing base (41ef19b) to head (7c5c55a).

Files with missing lines	Patch %	Lines
allowance_charge_parse.go	47.05%	16 Missing and 2 partials ⚠️
party_parse.go	68.75%	4 Missing and 1 partial ⚠️
settlement_parse.go	69.23%	4 Missing ⚠️
lines_parse.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           fix-buyer-reference      #34      +/-   ##
=======================================================
- Coverage                72.75%   71.54%   -1.22%     
=======================================================
  Files                       23       23              
  Lines                     1681     1743      +62     
=======================================================
+ Hits                      1223     1247      +24     
- Misses                     376      410      +34     
- Partials                    82       86       +4     

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