fix: revert Count==1 guard removal, keep bare dynamicRef as reference only

The Count==1 guard is correct: bare  (no siblings) creates an
OpenApiSchemaReference with Target resolution.  with structural
siblings (maxProperties, type, etc.) falls through to ParseMap as a plain
OpenApiSchema, preserving siblings. This matches the  behavior where
structural constraints coexist with the reference.

Reverts the sibling serialization changes since bare dynamic-only refs
carry no siblings. Removes two incorrect tests that expected annotation
siblings on a reference.

Author: aqeelat

src/Microsoft.OpenApi/Models/JsonSchemaReference.cs

@@ -423,7 +423,8 @@ public override void SerializeAsV31(IOpenApiWriter writer)
         if (IsDynamicRefOnly)
         {
             writer.WriteStartObject();
-            SerializeDynamicRefOnly(writer, (w, e) => e.SerializeAsV31(w));
+            writer.WriteProperty(OpenApiConstants.DynamicRef, DynamicRef);
+            writer.WriteExtensions(Extensions, OpenApiSpecVersion.OpenApi3_1);
             writer.WriteEndObject();
         }
         else
@@ -437,7 +438,8 @@ public override void SerializeAsV32(IOpenApiWriter writer)
         if (IsDynamicRefOnly)
         {
             writer.WriteStartObject();
-            SerializeDynamicRefOnly(writer, (w, e) => e.SerializeAsV32(w));
+            writer.WriteProperty(OpenApiConstants.DynamicRef, DynamicRef);
+            writer.WriteExtensions(Extensions, OpenApiSpecVersion.OpenApi3_1);
             writer.WriteEndObject();
         }
         else
@@ -457,16 +459,6 @@ protected override void SerializeAdditionalV32Properties(IOpenApiWriter writer)
         SerializeAdditionalV3XProperties(writer, (w, e) => e.SerializeAsV32(w), base.SerializeAdditionalV32Properties);
     }
 
-    private void SerializeDynamicRefOnly(IOpenApiWriter writer, Action<IOpenApiWriter, IOpenApiSerializable> serializeCallback)
-    {
-        // A dynamic-only reference is always a pure $dynamicRef object (the deserializer only sets
-        // IsDynamicRefOnly when the schema has no other keywords), so it carries no schema siblings.
-        // Serialize $dynamicRef (without $ref) plus any extensions. The serializeCallback is accepted
-        // for signature symmetry with the version-specific serialize paths.
-        _ = serializeCallback;
-        writer.WriteProperty(OpenApiConstants.DynamicRef, DynamicRef);
-        writer.WriteExtensions(Extensions, OpenApiSpecVersion.OpenApi3_1);
-    }
     private void SerializeAdditionalV3XProperties(IOpenApiWriter writer, Action<IOpenApiWriter, IOpenApiSerializable> serializeCallback, Action<IOpenApiWriter> baseSerializer)
     {
         if (Type != ReferenceType.Schema) throw new InvalidOperationException(

src/Microsoft.OpenApi/Reader/V31/OpenApiSchemaDeserializer.cs

@@ -468,22 +468,12 @@ public static IOpenApiSchema LoadSchema(JsonNode node, OpenApiDocument hostDocum
             return result;
         }
 
-        if (dynamicPointer != null)
+        if (dynamicPointer != null && jsonObject.Count == 1)
         {
             var anchorName = JsonNodeHelper.ExtractDynamicAnchorName(dynamicPointer);
             var result = new OpenApiSchemaReference(!string.IsNullOrEmpty(anchorName) ? anchorName! : dynamicPointer, hostDocument);
             result.Reference.DynamicRef = dynamicPointer;
             result.Reference.IsDynamicRefOnly = true;
-            result.Reference.SetMetadataFromJsonObject(jsonObject);
-
-            if (jsonObject.TryGetPropertyValue(OpenApiConstants.Defs, out var dynDefsNode) && dynDefsNode is JsonObject)
-            {
-                context.StartObject(OpenApiConstants.Defs);
-                if (dynDefsNode.CreateMap(LoadSchema, hostDocument, context) is { Count: > 0 } dynDefinitions)
-                    result.Reference.Definitions = dynDefinitions;
-                context.EndObject();
-            }
-
             return result;
         }
 

src/Microsoft.OpenApi/Reader/V32/OpenApiSchemaDeserializer.cs

@@ -468,22 +468,12 @@ public static IOpenApiSchema LoadSchema(JsonNode node, OpenApiDocument hostDocum
             return result;
         }
 
-        if (dynamicPointer != null)
+        if (dynamicPointer != null && jsonObject.Count == 1)
         {
             var anchorName = JsonNodeHelper.ExtractDynamicAnchorName(dynamicPointer);
             var result = new OpenApiSchemaReference(!string.IsNullOrEmpty(anchorName) ? anchorName! : dynamicPointer, hostDocument);
             result.Reference.DynamicRef = dynamicPointer;
             result.Reference.IsDynamicRefOnly = true;
-            result.Reference.SetMetadataFromJsonObject(jsonObject);
-
-            if (jsonObject.TryGetPropertyValue(OpenApiConstants.Defs, out var dynDefsNode) && dynDefsNode is JsonObject)
-            {
-                context.StartObject(OpenApiConstants.Defs);
-                if (dynDefsNode.CreateMap(LoadSchema, hostDocument, context) is { Count: > 0 } dynDefinitions)
-                    result.Reference.Definitions = dynDefinitions;
-                context.EndObject();
-            }
-
             return result;
         }
 

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiDynamicRefTests.cs

@@ -706,75 +706,4 @@ public async Task CreateShallowCopyPreservesValidationSiblings()
         Assert.NotNull(copy.AllOf);
         Assert.Single(copy.AllOf);
     }
-
-    [Fact]
-    public async Task DynamicRefWithAnnotationSiblingResolvesAndPreservesSibling()
-    {
-        var yaml =
-        """
-        openapi: 3.1.0
-        info:
-          title: Test
-          version: 1.0.0
-        paths: {}
-        components:
-          schemas:
-            Tree:
-              $dynamicAnchor: node
-              type: object
-              properties:
-                value:
-                  type: string
-                next:
-                  $dynamicRef: '#node'
-                  description: The next node in the tree
-        """;
-
-        var doc = await LoadDocumentAsync(yaml);
-
-        var tree = doc.Components.Schemas["Tree"];
-        var nextSchema = tree.Properties["next"];
-
-        var reference = Assert.IsType<OpenApiSchemaReference>(nextSchema);
-        Assert.True(reference.Reference.IsDynamicRefOnly);
-        Assert.NotNull(reference.Target);
-        Assert.Same(tree, reference.Target);
-        Assert.Equal("The next node in the tree", reference.Description);
-    }
-
-    [Fact]
-    public async Task DynamicRefWithDefsSiblingParsesDefs()
-    {
-        var yaml =
-        """
-        openapi: 3.1.0
-        info:
-          title: Test
-          version: 1.0.0
-        paths: {}
-        components:
-          schemas:
-            Tree:
-              $dynamicAnchor: node
-              type: object
-              properties:
-                next:
-                  $dynamicRef: '#node'
-                  $defs:
-                    extra:
-                      type: string
-        """;
-
-        var doc = await LoadDocumentAsync(yaml);
-
-        var tree = doc.Components.Schemas["Tree"];
-        var nextSchema = tree.Properties["next"];
-
-        var reference = Assert.IsType<OpenApiSchemaReference>(nextSchema);
-        Assert.True(reference.Reference.IsDynamicRefOnly);
-        Assert.NotNull(reference.Target);
-        Assert.Same(tree, reference.Target);
-        Assert.NotNull(reference.Definitions);
-        Assert.True(reference.Definitions.ContainsKey("extra"));
-    }
 }