Default missing Feedback-Type and Authentication-Results in failure reports (#332) (#831)

Some Exim/cPanel-based gateways send DMARC failure reports without a
machine-readable message/feedback-report part. parse_report_email()'s
plain-text fallback synthesizes a minimal feedback report with only
Arrival-Date and Source-IP, but the Elasticsearch/OpenSearch outputs
access feedback_type and authentication_results with hard key lookups,
so every such report was archived but never indexed, failing with
"Failure report missing required field: 'feedback_type'".

parse_failure_report() now defaults feedback_type to auth-failure
(RFC 5965 3.1) and authentication_results to None (RFC 6591 3.1) with
logged warnings, matching the existing handling of the REQUIRED
Auth-Failure and Identity-Alignment fields. Adds a sanitized sample
and a regression test asserting the sink-required keys are present.

tests/test_init.py

@@ -191,6 +191,18 @@ class Test(unittest.TestCase):
                 )
             print("Passed!")
 
+    def testFailureSampleWithoutFeedbackReportPart(self):
+        """A plain-text-only failure report (no message/feedback-report part)
+        must still contain every field the Elasticsearch/OpenSearch outputs
+        access with hard key lookups (issue #332)"""
+        sample_path = "samples/failure/exim_plain_text_only_no_arf_part.eml"
+        result = parsedmarc.parse_report_file(sample_path, offline=OFFLINE_MODE)
+        assert result["report_type"] == "failure"
+        report = cast(FailureReport, result["report"])
+        assert report["feedback_type"] == "auth-failure"
+        assert "authentication_results" in report
+        assert report["source"]["ip_address"] == "203.0.113.68"
+
     def testFailureReportBackwardCompat(self):
         """Test that old forensic function aliases still work"""
         self.assertIs(