fix: extract tax number, price basis quantity, spaced VAT IDs and contact email

- supplier/buyer: extract FC tax registration (BT-32/BT-49) into new
  tax_number field; previously only VA (USt-IdNr.) was read
- pflichtfelder: critical error only when neither vat_id nor tax_number
  is present (EN 16931 BR-CO-26)
- ustid: tolerate formatting whitespace inside VAT IDs (e.g. 'DE 140 978 617')
- line items: normalize unit price by basis quantity (BT-149), so a
  price per 100 units no longer inflates line totals by factor 100
- supplier email: also look in DefinedTradeContact (BT-42)
- rewrite xrechnung fallback test to assert only our own behavior,
  independent of factur-x version specifics

tests/test_validator.py

@@ -149,6 +149,58 @@ class TestValidatePflichtfelder:
         assert any(e.field == "supplier.vat_id" for e in errors)
         assert any(e.severity == "critical" for e in errors)
 
+    def test_tax_number_only_supplier_no_critical_error(self):
+        """Supplier with only a tax number (no VAT ID) should pass BR-CO-26."""
+        xml_data = XmlData(
+            invoice_number="INV001",
+            invoice_date="2024-01-15",
+            supplier=Supplier(
+                name="Werkstatt gGmbH",
+                vat_id=None,
+                tax_number="2281410000697",
+            ),
+            buyer=Buyer(name="Test Buyer AG"),
+            line_items=[
+                LineItem(
+                    position=1,
+                    description="Test Product",
+                    quantity=2.0,
+                    unit="HUR",
+                    unit_price=5.0,
+                    line_total=10.0,
+                    vat_rate=7.0,
+                )
+            ],
+            totals=Totals(line_total_sum=10.0, net=10.0, vat_total=0.7, gross=10.7),
+        )
+        errors = validate_pflichtfelder(xml_data)
+        assert not any(e.field == "supplier.vat_id" for e in errors)
+
+    def test_missing_vat_id_and_tax_number_critical(self):
+        """Supplier with neither VAT ID nor tax number stays critical."""
+        xml_data = XmlData(
+            invoice_number="INV001",
+            invoice_date="2024-01-15",
+            supplier=Supplier(name="Test Supplier GmbH", vat_id="", tax_number=""),
+            buyer=Buyer(name="Test Buyer AG"),
+            line_items=[
+                LineItem(
+                    position=1,
+                    description="Test Product",
+                    quantity=2.0,
+                    unit="HUR",
+                    unit_price=5.0,
+                    line_total=10.0,
+                    vat_rate=7.0,
+                )
+            ],
+            totals=Totals(line_total_sum=10.0, net=10.0, vat_total=0.7, gross=10.7),
+        )
+        errors = validate_pflichtfelder(xml_data)
+        assert any(e.field == "supplier.vat_id" for e in errors)
+        assert any(e.severity == "critical" for e in errors)
+        assert any("tax number" in e.message for e in errors)
+
     def test_missing_buyer_name_critical(self):
         """Missing buyer.name should produce critical error."""
         xml_data = XmlData(
@@ -720,6 +772,11 @@ class TestValidateUstid:
         error = validate_ustid("DE123456789")
         assert error is None
 
+    def test_valid_german_vat_id_with_spaces(self):
+        """German VAT ID with formatting spaces (e.g. 'DE 140 978 617') passes."""
+        error = validate_ustid("DE 140 978 617")
+        assert error is None
+
     def test_valid_austrian_vat_id(self):
         """Valid Austrian VAT ID should pass."""
         error = validate_ustid("ATU12345678")