Harden diagnostic handling and warning presentation

internal/framework/pipeline/validation_executor_test.go

@@ -196,6 +196,19 @@ func TestExecuteValidationChainReturnsFrameworkAndCancellationErrors(t *testing.
 	}
 }
 
+func TestExecuteValidationChainRejectsInvalidDiagnosticsWithoutRetry(t *testing.T) {
+	chain := validationChain(validationSpec("validator", contracts.ExecutionClassLLMBacked, 2))
+	calls := 0
+	_, err := executeValidationChain(context.Background(), chain, func(context.Context, preparedValidator, int) (validationInvocation, error) {
+		calls++
+		return validationInvocation{result: contracts.ValidationResult{Approved: true, Diagnostics: []contracts.ProducerDiagnostic{{}}}}, nil
+	})
+	var frameworkErr validationFrameworkError
+	if !errors.As(err, &frameworkErr) || calls != 1 || !strings.Contains(err.Error(), "validator \"validator\" returned invalid diagnostics") {
+		t.Fatalf("error = %v calls = %d, want one fatal invalid-diagnostics result", err, calls)
+	}
+}
+
 type validationStep struct {
 	result     contracts.ValidationResult
 	invocation validationInvocation