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>

src/validator.py

@@ -4,6 +4,7 @@ import re
 import time
 from typing import Any
 
+from pydantic import ValidationError
 from src.models import (
     ErrorDetail,
     ValidateRequest,
@@ -275,7 +276,34 @@ def validate_invoice(request: ValidateRequest) -> ValidationResult:
             validation_time_ms=0,
         )
 
-    xml_data = XmlData(**request.xml_data)
+    try:
+        xml_data = XmlData(**request.xml_data)
+    except ValidationError as e:
+        # Convert Pydantic validation errors to ValidationResult
+        validation_errors = []
+        for error in e.errors():
+            validation_errors.append(
+                ErrorDetail(
+                    check="schema_validation",
+                    field=error["loc"][0] if error["loc"] else None,
+                    error_code=error["type"],
+                    message=error["msg"],
+                    severity="critical",
+                )
+            )
+        return ValidationResult(
+            is_valid=False,
+            errors=validation_errors,
+            warnings=[],
+            summary={
+                "total_checks": 1,
+                "checks_passed": 0,
+                "checks_failed": 1,
+                "critical_errors": len(validation_errors),
+                "warnings": 0,
+            },
+            validation_time_ms=int((time.time() - start_time) * 1000),
+        )
 
     # Run requested checks
     for check_name in request.checks: