fix: handle 2.x message traits without headers

The 2.x writer threw NullReferenceException for a message trait without
headers because it read Headers.SchemaFormat unconditionally. The 2.x reader
threw too when schemaFormat appeared without headers, or before them, and a
later headers field discarded an earlier schemaFormat.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: bielu

src/ByteBard.AsyncAPI.Readers/V2/AsyncApiMessageTraitDeserializer.cs

@@ -8,9 +8,9 @@ internal static partial class AsyncApiV2Deserializer
     {
         private static FixedFieldMap<AsyncApiMessageTrait> messageTraitFixedFields = new()
         {
-            { "headers", (a, n) => { a.Headers = new AsyncApiMultiFormatSchema { Schema = AsyncApiJsonSchemaDeserializer.LoadSchema(n) }; } },
+            { "headers", (a, n) => { (a.Headers ??= new AsyncApiMultiFormatSchema()).Schema = AsyncApiJsonSchemaDeserializer.LoadSchema(n); } },
             { "correlationId", (a, n) => { a.CorrelationId = LoadCorrelationId(n); } },
-            { "schemaFormat", (a, n) => { a.Headers.SchemaFormat = n.GetScalarValue(); } },
+            { "schemaFormat", (a, n) => { (a.Headers ??= new AsyncApiMultiFormatSchema()).SchemaFormat = n.GetScalarValue(); } },
             { "contentType", (a, n) => { a.ContentType = n.GetScalarValue(); } },
             { "name", (a, n) => { a.Name = n.GetScalarValue(); } },
             { "title", (a, n) => { a.Title = n.GetScalarValue(); } },

src/ByteBard.AsyncAPI/Models/AsyncApiMessageTrait.cs

@@ -76,9 +76,9 @@ public virtual void SerializeV2(IAsyncApiWriter writer)
 
             writer.WriteStartObject();
             //writer.WriteOptionalProperty(AsyncApiConstants.MessageId, this.MessageId);
-            writer.WriteOptionalObject(AsyncApiConstants.Headers, this.Headers, (w, h) => h.Schema.SerializeV2(w));
+            writer.WriteOptionalObject(AsyncApiConstants.Headers, this.Headers?.Schema, (w, h) => h.SerializeV2(w));
             writer.WriteOptionalObject(AsyncApiConstants.CorrelationId, this.CorrelationId, (w, c) => c.SerializeV2(w));
-            writer.WriteOptionalProperty(AsyncApiConstants.SchemaFormat, this.Headers.SchemaFormat);
+            writer.WriteOptionalProperty(AsyncApiConstants.SchemaFormat, this.Headers?.SchemaFormat);
             writer.WriteOptionalProperty(AsyncApiConstants.ContentType, this.ContentType);
             writer.WriteOptionalProperty(AsyncApiConstants.Name, this.Name);
             writer.WriteOptionalProperty(AsyncApiConstants.Title, this.Title);
@@ -100,7 +100,7 @@ public virtual void SerializeV3(IAsyncApiWriter writer)
             }
 
             writer.WriteStartObject();
-            writer.WriteOptionalObject(AsyncApiConstants.Headers, this.Headers, (w, h) => h.SerializeV3(w));
+            writer.WriteOptionalObject(AsyncApiConstants.Headers, this.Headers?.Schema == null ? null : this.Headers, (w, h) => h.SerializeV3(w));
             writer.WriteOptionalObject(AsyncApiConstants.CorrelationId, this.CorrelationId, (w, c) => c.SerializeV3(w));
             writer.WriteOptionalProperty(AsyncApiConstants.ContentType, this.ContentType);
             writer.WriteOptionalProperty(AsyncApiConstants.Name, this.Name);

test/ByteBard.AsyncAPI.Tests/Models/AsyncApiMessage_Should.cs

@@ -87,6 +87,52 @@ public void V2_AsyncApiMessage_WithUnsupportedSchemaFormat_DeserializesWithError
             diagnostic.Errors.First().Message.Should().StartWith("Could not deserialize Payload. Supported formats are");
         }
 
+        [Test]
+        public void V2_AsyncApiMessageTrait_WithoutHeaders_Serializes()
+        {
+            var expected =
+                """
+                name: TraitName
+                """;
+
+            var trait = new AsyncApiMessageTrait { Name = "TraitName" };
+
+            var actual = trait.SerializeAsYaml(AsyncApiVersion.AsyncApi2_0);
+
+            actual.Should().BePlatformAgnosticEquivalentTo(expected);
+        }
+
+        [Test]
+        public void V2_AsyncApiMessageTrait_WithSchemaFormatBeforeHeaders_Deserializes()
+        {
+            var input =
+                """
+                schemaFormat: application/vnd.aai.asyncapi+json;version=2.6.0
+                headers:
+                  type: object
+                """;
+
+            var trait = new AsyncApiStringReader().ReadFragment<AsyncApiMessageTrait>(input, AsyncApiVersion.AsyncApi2_0, out var diagnostic);
+
+            diagnostic.Errors.Should().BeEmpty();
+            trait.Headers.SchemaFormat.Should().Be("application/vnd.aai.asyncapi+json;version=2.6.0");
+            trait.Headers.Schema.Should().BeOfType<AsyncApiJsonSchema>();
+        }
+
+        [Test]
+        public void V2_AsyncApiMessageTrait_WithSchemaFormatAndNoHeaders_Deserializes()
+        {
+            var input =
+                """
+                schemaFormat: application/vnd.aai.asyncapi+json;version=2.6.0
+                """;
+
+            var trait = new AsyncApiStringReader().ReadFragment<AsyncApiMessageTrait>(input, AsyncApiVersion.AsyncApi2_0, out var diagnostic);
+
+            diagnostic.Errors.Should().BeEmpty();
+            trait.SerializeAsYaml(AsyncApiVersion.AsyncApi2_0).Should().BePlatformAgnosticEquivalentTo(input);
+        }
+
         [Test]
         public void V2_AsyncApiMessage_WithNoSchemaFormat_DoesNotSerializeSchemaFormat()
         {