Allow 1-cent rounding on line VAT, use line totals as the VAT base, fix the ITEM_BASED check

[ahmedm10]

Problem

When the gross price of a line is fixed and net and VAT are derived from it (common for hotel and event invoices in Germany), the VAT can be one cent off net × rate. Example: 8 nights at 52.00 gross → net 388.79, VAT 27.21, while 388.79 × 7 % rounds to 27.22.

LineItem#valid? requires exact equality, so such an invoice is refused, and the only way through is to let the gem recalculate the VAT (:VERTICAL) — which makes the XML say 27.22 / 416.01 while the human-readable invoice says 27.21 / 416.00. EN 16931 itself tolerates this rounding (BR-CO-17).

Second, the VAT base (BT-116) is summed from net_amount × billed_quantity rather than from the line totals. With unit prices stored to four decimals the two drift apart: two lines of 120 × 41.6667 (5,000.00 each) give a base of 10000.008, written as 10000.01 next to line totals of 5000.00 + 5000.00 — the XML then violates BR-CO-10 and BR-S-08. Once the line totals are rounded to cents, the invoice is refused instead ("Base amount and summed tax base amount deviate").

Third, the :ITEM_BASED check in Invoice#valid? compares tax_amount with the summed line tax without converting either side, so a total passed as a string never matches. It records an error message but does not return false, so the invoice is still reported as valid — with a misleading entry in errors.

Change

• LineItem#valid?: a line's VAT may differ from net × rate by at most one cent (LINE_TAX_ROUNDING_TOLERANCE). Larger differences are still refused. The invoice-level checks stay exact.
• Invoice#taxes: the VAT base is the sum of the line totals (LineItem#signed_charge_amount — negative for a negative quantity, the same rule #valid? already uses when it checks that the lines add up to the basis).
• Invoice#valid? (:ITEM_BASED): compares the values as BigDecimal, returns false on a mismatch, and prints the amounts in plain notation.

Tests

Five new tests in test/invoice_test.rb: the gross-leading invoice is accepted and its XML carries 27.21 / 416.00; a line two cents off is still refused; item-based totals passed as strings leave no error; an item-based total that differs from its lines is refused; two lines of 120 × 41.6667 give a VAT base of 10000.00. The full suite passes.

Related

The VAT-base change is the same idea as #45 (tax base from charge_amount), keeping the sign for negative quantities; #45's contact fields and to_xml changes are not included. It also covers what #51 addresses: summing the line totals means each line is already rounded before the sum.