test: cover 3.x message trait headers

Adds 3.x counterparts to the 2.x message trait tests, so the change to the
3.x writer is shown not to regress: a trait without headers, a trait with
headers, and a 2.x trait with schemaFormat but no headers upgraded to 3.x.

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

Author: bielu

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

@@ -133,6 +133,63 @@ public void V2_AsyncApiMessageTrait_WithSchemaFormatAndNoHeaders_Deserializes()
             trait.SerializeAsYaml(AsyncApiVersion.AsyncApi2_0).Should().BePlatformAgnosticEquivalentTo(input);
         }
 
+        [Test]
+        public void V3_AsyncApiMessageTrait_WithoutHeaders_Serializes()
+        {
+            var expected =
+                """
+                name: TraitName
+                """;
+
+            var trait = new AsyncApiMessageTrait { Name = "TraitName" };
+
+            var actual = trait.SerializeAsYaml(AsyncApiVersion.AsyncApi3_0);
+
+            actual.Should().BePlatformAgnosticEquivalentTo(expected);
+        }
+
+        [Test]
+        public void V3_AsyncApiMessageTrait_WithHeaders_Reserializes()
+        {
+            var input =
+                """
+                headers:
+                  type: object
+                name: TraitName
+                """;
+
+            // The 3.x reader fills in the default schema format, so the headers come back in the explicit multi-format form.
+            var expected =
+                """
+                headers:
+                  schemaFormat: application/vnd.aai.asyncapi+json;version=3.0.0
+                  schema:
+                    type: object
+                name: TraitName
+                """;
+
+            var trait = new AsyncApiStringReader().ReadFragment<AsyncApiMessageTrait>(input, AsyncApiVersion.AsyncApi3_0, out var diagnostic);
+
+            diagnostic.Errors.Should().BeEmpty();
+            trait.Headers.Schema.Should().BeOfType<AsyncApiJsonSchema>();
+            trait.SerializeAsYaml(AsyncApiVersion.AsyncApi3_0).Should().BePlatformAgnosticEquivalentTo(expected);
+        }
+
+        [Test]
+        public void V2_AsyncApiMessageTrait_WithSchemaFormatAndNoHeaders_UpgradesToV3WithoutHeaders()
+        {
+            var input =
+                """
+                schemaFormat: application/vnd.aai.asyncapi+json;version=2.6.0
+                name: TraitName
+                """;
+
+            var trait = new AsyncApiStringReader().ReadFragment<AsyncApiMessageTrait>(input, AsyncApiVersion.AsyncApi2_0, out var diagnostic);
+
+            diagnostic.Errors.Should().BeEmpty();
+            trait.SerializeAsYaml(AsyncApiVersion.AsyncApi3_0).Should().BePlatformAgnosticEquivalentTo("name: TraitName");
+        }
+
         [Test]
         public void V2_AsyncApiMessage_WithNoSchemaFormat_DoesNotSerializeSchemaFormat()
         {