Make pay.DueDate amount optional

[migueltorresvalls]

Relax GOBL-PAY-DUEDATE-02 so a due date without an amount is valid.

Use case: when parsing CII documents into GOBL, ram:PartialPaymentAmount and ram:PartialPaymentPercent are optional fields that gobl.cii treats as parse-only — they are not part of the EN16931 / Factur-X / ZUGFeRD schemas, so they're read when present but never emitted. A payment term carrying only a ram:DueDateDateTime is therefore a perfectly valid source document, yet the converted GOBL invoice failed validation: DueDate.Amount was a non-pointer num.Amount, so the missing amount materialized as zero, CalculateDues rescaled it to 0.00, and GOBL-PAY-DUEDATE-02 ("amount must not be zero") rejected the whole document. The fix stops inventing a zero amount for due dates that never declared one, instead of loosening the check on amounts that are actually set.

How gobl.ubl handles the same situation: instead of leaving the amount unset, it assumes a single due date covers the full amount (payment_parse.go), which CalculateDues then turns into the payable total:

// If there's only one due date, set its percent to 100
if payment.Terms != nil && len(payment.Terms.DueDates) == 1 {
    percent, err := num.PercentageFromString("100%")
    if err != nil {
        return err
    }
    payment.Terms.DueDates[0].Percent = &percent
}

invopop/gobl.cii#88 (draft) ports that same behavior to gobl.cii.

Changes:

• pay: DueDate.Amount is now *num.Amount with omitempty, matching the other optional amounts; an absent amount stays absent
• pay: CalculateDues leaves due dates without an amount or percent untouched; percent-derived amounts are still calculated and rescaled
• GOBL-PAY-DUEDATE-02 now only fires for an explicitly set zero amount (num.NotZero already skips nil)
• Regenerated data/schemas/pay/terms.json: amount no longer required on pay.DueDate

Notes:

• Up for discussion how we handle this: it's not clear we should always be assuming 100% at parse time (the source document doesn't state it), nor whether GOBL should accept amount-less due dates instead — or both, since the 100% assumption only covers the single-due-date case and multiple due dates without amounts would still need this relaxation.
• gobl.cii assigns dd.Amount from a parsed value; it needs a one-line pointer change when bumping to a gobl version including this.

Generated with Claude Code — https://claude.ai/code/ecc6fcbb-f34f-4ab9-bc09-90ec8cc9c4ae

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.99%. Comparing base (9cbe45b) to head (e5af1b1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #951      +/-   ##
==========================================
- Coverage   95.02%   94.99%   -0.03%     
==========================================
  Files         327      321       -6     
  Lines       17773    17338     -435     
==========================================
- Hits        16888    16471     -417     
+ Misses        546      539       -7     
+ Partials      339      328      -11     

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