fix: resolve test failures and update flake.nix for factur-x source distribution

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

tests/test_extractor.py

@@ -46,20 +46,18 @@ class TestFileSizeValidation:
     """Test file size validation in extract_zugferd()."""
 
     def test_file_size_limit_exactly_10mb(self):
-        """Test PDF exactly at 10MB limit is accepted."""
+        """Test PDF exactly at 10MB limit passes size check but fails PDF parsing."""
         from src.extractor import extract_zugferd, ExtractionError
 
-        """Test PDF exactly at 10MB limit is accepted."""
-        from src.extractor import extract_zugferd
-
         # 10MB = 10 * 1024 * 1024 bytes
         large_pdf = b"X" * (10 * 1024 * 1024)
 
-        # Should raise file_too_large error
+        # 10MB exactly is allowed (not > 10MB), but invalid PDF data causes parse error
         with pytest.raises(ExtractionError) as exc_info:
             extract_zugferd(large_pdf)
 
-        assert exc_info.value.error_code == "file_too_large"
+        # Should fail PDF parsing, not file size check
+        assert exc_info.value.error_code in ["corrupt_pdf", "invalid_pdf"]
 
     def test_file_size_limit_10mb_plus_one_byte(self):
         """Test PDF one byte over 10MB limit is rejected."""