fix(policies): graceful fallback for structured finding violations

When a policy declares finding_type but some evaluation branches return
plain-string violations, fall back to treating them as regular string
violations instead of failing the entire policy evaluation. Structured
violations from other branches are still validated normally.

Signed-off-by: Miguel Martinez <miguel@chainloop.dev>
Signed-off-by: Miguel Martinez Trivino <miguel@chainloop.dev>

Author: migmartri

pkg/policies/policies.go

@@ -758,6 +758,7 @@ func engineEvaluationsToAPIViolations(results []*engine.EvaluationResult, findin
 	res := make([]*v12.PolicyEvaluation_Violation, 0)
 	var warnings []string
 	warnedNoFindingType := false
+	warnedNoStructuredData := false
 
 	for _, r := range results {
 		for _, v := range r.Violations {
@@ -780,7 +781,14 @@ func engineEvaluationsToAPIViolations(results []*engine.EvaluationResult, findin
 				}
 
 			case findingType != "" && !hasStructuredData:
-				return nil, nil, fmt.Errorf("declares finding_type %q but violation is not a structured object", findingType)
+				// Policy declares a finding type but this violation is a plain string.
+				// This can happen when some evaluation branches do not support structured output yet.
+				// Fall back to treating it as a regular string violation without the typed finding.
+				if !warnedNoStructuredData {
+					warnings = append(warnings,
+						fmt.Sprintf("policy declares finding_type %q but some violations are plain strings — structured finding data will not be available for those", findingType))
+					warnedNoStructuredData = true
+				}
 
 			case findingType != "" && hasStructuredData:
 				finding, err := findings.ValidateFinding(findingType, v.RawFinding)

pkg/policies/policies_test.go

@@ -1526,12 +1526,47 @@ func (s *testSuite) TestEngineEvaluationsToAPIViolationsBehaviorMatrix() {
 			},
 		},
 		{
-			name:        "finding_type + string violations - error",
+			name:        "finding_type + string violations - fallback with warning",
 			findingType: "VULNERABILITY",
 			violations: []*engine.PolicyViolation{
 				{Subject: "p1", Violation: "plain string"},
 			},
-			wantErr: "declares finding_type",
+			wantViolations: 1,
+			wantWarnings:   1,
+			checkFn: func(violations []*v1.PolicyEvaluation_Violation) {
+				// The violation is kept as a plain string, no structured finding
+				s.Equal("plain string", violations[0].GetMessage())
+				s.Nil(violations[0].GetVulnerability())
+			},
+		},
+		{
+			name:        "finding_type + mixed string and structured violations - per-violation handling",
+			findingType: "VULNERABILITY",
+			violations: []*engine.PolicyViolation{
+				{Subject: "p1", Violation: "plain string fallback"},
+				{Subject: "p2", Violation: "vuln found", RawFinding: map[string]any{
+					"message": "vuln found", "external_id": "CVE-2024-1234",
+					"package_purl": "pkg:golang/example.com/lib@v1.0.0", "severity": "HIGH",
+				}},
+				{Subject: "p3", Violation: "another plain string"},
+			},
+			wantViolations: 3,
+			wantWarnings:   1, // deduplicated warning
+			checkFn: func(violations []*v1.PolicyEvaluation_Violation) {
+				// First violation: plain string, no structured finding
+				s.Equal("plain string fallback", violations[0].GetMessage())
+				s.Nil(violations[0].GetVulnerability())
+
+				// Second violation: structured, validated
+				f := violations[1].GetVulnerability()
+				s.Require().NotNil(f)
+				s.Equal("CVE-2024-1234", f.GetExternalId())
+				s.Equal("HIGH", f.GetSeverity())
+
+				// Third violation: plain string, no structured finding
+				s.Equal("another plain string", violations[2].GetMessage())
+				s.Nil(violations[2].GetVulnerability())
+			},
 		},
 		{
 			name:        "finding_type + valid structured violations - validates and sets oneof",