Make SnapshotModelProcessor idempotent. (#35543)

Fixes #35146

Author: AndriySvyryd

src/EFCore.Design/Migrations/Internal/SnapshotModelProcessor.cs

@@ -52,7 +52,9 @@ public SnapshotModelProcessor(
     /// </summary>
     public virtual IModel? Process(IReadOnlyModel? model, bool resetVersion = false)
     {
-        if (model == null)
+        if (model == null
+            || model is not Model mutableModel
+            || mutableModel.IsReadOnly)
         {
             return null;
         }
@@ -79,13 +81,10 @@ public SnapshotModelProcessor(
             }
         }
 
-        if (model is IMutableModel mutableModel)
+        mutableModel.RemoveAnnotation("ChangeDetector.SkipDetectChanges");
+        if (resetVersion)
         {
-            mutableModel.RemoveAnnotation("ChangeDetector.SkipDetectChanges");
-            if (resetVersion)
-            {
-                mutableModel.SetProductVersion(ProductInfo.GetVersion());
-            }
+            mutableModel.SetProductVersion(ProductInfo.GetVersion());
         }
 
         return _modelRuntimeInitializer.Initialize((IModel)model, designTime: true, validationLogger: null);

test/EFCore.Design.Tests/Migrations/Design/SnapshotModelProcessorTest.cs

@@ -43,7 +43,8 @@ public void Updates_provider_annotations_on_model()
 
             var reporter = new TestOperationReporter();
 
-            new SnapshotModelProcessor(reporter, DummyModelRuntimeInitializer.Instance).Process(model);
+            var processor = new SnapshotModelProcessor(reporter, DummyModelRuntimeInitializer.Instance);
+            processor.Process(model);
 
             AssertAnnotations(model);
             AssertAnnotations(entityType);
@@ -54,6 +55,8 @@ public void Updates_provider_annotations_on_model()
             AssertAnnotations(nav2);
             AssertAnnotations(index);
 
+            Assert.Same(model, processor.Process(model));
+
             Assert.Empty(reporter.Messages);
         }