fix(tests): use ReferenceHandler.Preserve for STJ snapshot with $dynamicRef

The Serialize_DoesNotMutateDom tests use System.Text.Json to snapshot
the DOM before and after native serialization. Bare $dynamicRef schemas
now deserialize as OpenApiSchemaReference whose computed getters expose
the resolved target's properties, creating reflection cycles for
recursive dynamic refs. ReferenceHandler.Preserve lets STJ handle
cycles via $id/$ref metadata while still detecting DOM mutations.

Author: aqeelat

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

@@ -27,6 +27,7 @@ public async Task Serialize_DoesNotMutateDom(OpenApiSpecVersion version)
             // Act: Serialize using System.Text.Json
             var options = new JsonSerializerOptions
             {
+                ReferenceHandler = System.Text.Json.Serialization.ReferenceHandler.Preserve,
                 Converters =
                 {
                     new HttpMethodOperationDictionaryConverter()

test/Microsoft.OpenApi.Readers.Tests/V32Tests/OpenApiDocumentSerializationTests .cs

@@ -28,6 +28,7 @@ public async Task Serialize_DoesNotMutateDom(OpenApiSpecVersion version)
             // Act: Serialize using System.Text.Json
             var options = new JsonSerializerOptions
             {
+                ReferenceHandler = System.Text.Json.Serialization.ReferenceHandler.Preserve,
                 Converters =
                 {
                     new HttpMethodOperationDictionaryConverter()