fix(extractor): detect xrechnung XML attachments

tests/test_extractor.py

@@ -7,6 +7,8 @@ then implementation makes them pass (GREEN phase).
 import pytest
 import base64
 
+from pypdf import PdfReader, PdfWriter
+
 
 class TestExtractionError:
     """Test ExtractionError exception class."""
@@ -299,3 +301,60 @@ class TestZUGFeRDProfileVariations:
         # Profile should be detected (BASIC, BASICWL, etc.)
         assert result.zugferd_profil is not None
         assert result.xml_data is not None
+
+
+class TestXRechnungExtraction:
+    """Test extraction from XRechnung PDFs with non-standard filenames."""
+
+    def test_xrechnung_by_xml_extension_fallback(self):
+        """Test XRechnung with 'xrechnung.xml' filename is detected via fallback.
+
+        This test verifies the fallback mechanism when factur-x library
+        doesn't recognize non-standard filenames like 'xrechnung.xml'.
+        We create a synthetic PDF with a non-standard attachment name to
+        exercise the fallback code path.
+        """
+        from io import BytesIO
+
+        from src.extractor import extract_zugferd, _find_xml_attachment_fallback
+
+        # Load valid XML from existing test fixture
+        with open("tests/fixtures/validXRechnung.pdf", "rb") as f:
+            orig_reader = PdfReader(f)
+            xml_content = None
+            for att in orig_reader.attachment_list:
+                xml_content = att.content
+                break
+            if xml_content is None:
+                pytest.fail("Test fixture has no XML attachment")
+
+        # Create a new PDF with a non-standard attachment name
+        pdf_writer = PdfWriter()
+        pdf_writer.add_blank_page(width=72, height=72)
+        pdf_writer.add_attachment("xrechnung.xml", xml_content)
+
+        output = BytesIO()
+        pdf_writer.write(output)
+        pdf_bytes = output.getvalue()
+
+        # Verify factur-x does NOT recognize it (returns False/None)
+        from facturx import get_xml_from_pdf
+
+        facturx_result = get_xml_from_pdf(pdf_bytes, check_xsd=False)
+        assert facturx_result[0] is False, "factur-x should not recognize xrechnung.xml"
+
+        # Verify our fallback function DOES find it
+        fallback_result = _find_xml_attachment_fallback(pdf_bytes)
+        assert fallback_result[0] == "xrechnung.xml", (
+            "Fallback should find xrechnung.xml"
+        )
+        assert fallback_result[1] is not None, "Fallback should return XML content"
+
+        # Verify full extraction works via fallback
+        result = extract_zugferd(pdf_bytes)
+
+        assert result.is_zugferd is True
+        assert result.xml_raw is not None
+        assert result.xml_data is not None
+        assert result.xml_data.invoice_number is not None
+        assert result.extraction_meta.xml_attachment_name == "xrechnung.xml"