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

src/validator.py

@@ -19,13 +19,13 @@ def validate_pflichtfelder(xml_data: XmlData) -> list[ErrorDetail]:
     """Check required fields are present."""
     errors = []
 
-    def add_error(field: str, severity: str) -> None:
+    def add_error(field: str, severity: str, message: str | None = None) -> None:
         errors.append(
             ErrorDetail(
                 check="pflichtfelder",
                 field=field,
                 error_code="missing_required",
-                message=f"Required field '{field}' is missing or empty",
+                message=message or f"Required field '{field}' is missing or empty",
                 severity=severity,
             )
         )
@@ -40,8 +40,20 @@ def validate_pflichtfelder(xml_data: XmlData) -> list[ErrorDetail]:
     if not xml_data.supplier.name or not xml_data.supplier.name.strip():
         add_error("supplier.name", "critical")
 
-    if not xml_data.supplier.vat_id or not xml_data.supplier.vat_id.strip():
-        add_error("supplier.vat_id", "critical")
+    # EN 16931 BR-CO-26: seller identified by VAT ID (BT-31) OR tax number (BT-32)
+    has_vat_id = bool(xml_data.supplier.vat_id and xml_data.supplier.vat_id.strip())
+    has_tax_number = bool(
+        xml_data.supplier.tax_number and xml_data.supplier.tax_number.strip()
+    )
+    if not has_vat_id and not has_tax_number:
+        add_error(
+            "supplier.vat_id",
+            "critical",
+            message=(
+                "Supplier must have a VAT ID (supplier.vat_id) "
+                "or a tax number (supplier.tax_number)"
+            ),
+        )
 
     if not xml_data.buyer.name or not xml_data.buyer.name.strip():
         add_error("buyer.name", "critical")
@@ -163,7 +175,8 @@ def validate_ustid(vat_id: str) -> ErrorDetail | None:
             severity="critical",
         )
 
-    vat_id = vat_id.strip()
+    # Whitespace inside VAT IDs is formatting only (e.g. "DE 140 978 617")
+    vat_id = re.sub(r"\s+", "", vat_id)
 
     # German VAT ID: DE followed by 9 digits
     if vat_id.startswith("DE"):