fix(api): suppress historical_primary_product_mismatch when reg_2 is empty

Bug #10: When an order has only one registration number (reg_2 is empty)
but the reg historically appeared in tractor-trailer bookings, the
'historical_primary_product_mismatch' flag falsely suggests that the
current tractor-only billing is incorrect. The operator (Sarah #10714)
correctly expects that billing only a tractor is the right action when
there's no trailer to bill.

This change suppresses the historical_primary_product_mismatch flag for
rows where reg_2 is empty, matching the operator expectation. The
underlying historical SQL already filters by reg_1 only; a follow-up
could narrow the history to orders with matching reg_2 patterns, but for
the reported false-positive this fix is sufficient.

- invoice_period_flag_service.php: add o.reg_2 to the row projection
  and skip the historical mismatch loop when the current order's reg_2
  is empty.
- InvoicePeriodFlagServiceTest.php: include reg_2 in the existing
  historical mismatch test fixture so it still exercises the flag path.

services/nginx/app/classes/invoice_period_flag_service.php

@@ -758,6 +758,7 @@ class invoice_period_flag_service
                 o.department_id,
                 d.custom_pricing_only AS department_custom_pricing_only,
                 o.reg_1,
+                o.reg_2,
                 o.invoice_collection_id,
                 o.wash_id,
                 o.safety_seal,
@@ -1303,6 +1304,16 @@ class invoice_period_flag_service
             if ($reg === '' || !isset($history[$reg])) {
                 continue;
             }
+            // When the current order only has one registration number (reg_2 is empty),
+            // the historical "usual product" may include a trailer component (e.g.
+            // "Tractor-trailer wash") because the reg previously only appeared in
+            // tractor-trailer bookings. Billing just the tractor in that case is correct
+            // and matches the operator's expectation (bug #10, Sarah #10714). The mismatch
+            // is therefore uninformative when reg_2 is empty, so suppress the flag.
+            $reg2 = trim((string)($row['reg_2'] ?? ''));
+            if ($reg2 === '') {
+                continue;
+            }
             $expectedProductId = (int)$history[$reg]['product_id'];
             if ($this->primaryVehicleProductsMatch(
                 (int)$row['product_id'],

services/nginx/app/tests/Unit/Invoicing/InvoicePeriodFlagServiceTest.php

@@ -464,6 +464,7 @@ it('does not flag interior wash variants as historical primary product mismatche
             'invoice_collection_id' => 16912,
             'department_id' => 7,
             'reg_1' => 'CN96636',
+            'reg_2' => 'AB12345',
             'is_wash' => 1,
             'related_item_id' => 0,
             'order_created_at' => '2026-05-11 08:05:21',