Change NSB0018 saga diagnostic from Info to Error

Co-authored-by: danielmarbach <174258+danielmarbach@users.noreply.github.com>

Author: Copilot

src/NServiceBus.Core.Analyzer.Tests.Common/Sagas/SagaAnalyzerTests.cs

@@ -11,7 +11,7 @@ namespace NServiceBus.Core.Analyzer.Tests.Sagas;
 public class SagaAnalyzerTests : AnalyzerTestFixture<SagaAnalyzer>
 {
     [Test]
-    public Task InfoDiagnosticForSingleOldMapping()
+    public Task ErrorDiagnosticForSingleOldMapping()
     {
         var source =
 @"using System;

src/NServiceBus.Core.Analyzer/Sagas/SagaDiagnostics.cs

@@ -132,10 +132,10 @@ static class SagaDiagnostics
 
         public static readonly DiagnosticDescriptor SagaMappingExpressionCanBeRewritten = new(
             id: DiagnosticIds.SagaMappingExpressionCanBeRewritten,
-            title: "Saga mapping expressions can be simplified",
-            messageFormat: "This saga mapping expression can be rewritten using mapper.MapSaga(…).ToMessage<T>(…) syntax which avoids duplicate .ToSaga(…) expressions.",
+            title: "Saga mapping expressions must be rewritten",
+            messageFormat: "This saga mapping expression must be rewritten using mapper.MapSaga(…).ToMessage<T>(…) syntax. Use the code fix to transition to the new syntax.",
             category: DiagnosticCategory,
-            defaultSeverity: DiagnosticSeverity.Info,
+            defaultSeverity: DiagnosticSeverity.Error,
             isEnabledByDefault: true);
     }
 }