Centralize validator classification and malformed output handling

internal/framework/proposal_generation/generate_test.go

@@ -272,6 +272,23 @@ func TestGenerateCandidatesMalformedStructuredOutputReturnsWarning(t *testing.T)
 	}
 }
 
+func TestGenerateCandidatesProviderMalformedEnvelopeReturnsWarning(t *testing.T) {
+	client := &fakeStructuredClient{err: errors.New("provider response missing choices")}
+	req := defaultRequest(t)
+	req.LLMClient = client
+
+	result, err := GenerateCandidates(context.Background(), req)
+	if err != nil {
+		t.Fatalf("expected malformed provider envelope to downgrade to warning, got %v", err)
+	}
+	if len(result.Corrections) != 0 || len(result.Enriched) != 0 {
+		t.Fatalf("expected no proposals on malformed response, got %+v", result)
+	}
+	if len(result.Warnings) != 1 || result.Warnings[0].ReasonCode != "proposal_response_malformed" {
+		t.Fatalf("unexpected warnings: %+v", result.Warnings)
+	}
+}
+
 func TestGenerateCandidatesDeterministicIndexAssignment(t *testing.T) {
 	baseResponse := StructuredCorrectionSet{
 		Corrections: []StructuredCorrectionProposal{