fix: read 2.x Avro payloads in the local reference fallback

Adds LocalAvroPayloadReference_NotInWorkspace_ResolvesPayload for 2.6.0
and 3.0.0, as requested in review.

The 2.x case failed. For a 2.x document, the whole-document stream that
the local fallback reads is the upgraded document serialized as 3.0, so
an Avro payload appears there as a Multi Format Schema Object
({ schemaFormat, schema }). The 2.x loader read it as a JSON Schema and
reported "schemaFormat/schema is not a valid property". The 2.x reader
now uses the 3.x Multi Format Schema loader for that fallback.

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

Author: bielu

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

@@ -73,14 +73,6 @@ public static IAsyncApiSchema LoadAvroPayload(ParseNode n)
             return LoadPayload(n, "application/vnd.apache.avro");
         }
 
-        /// <summary>
-        /// 2.x has no Multi Format Schema Object; the schema format lives on the message, so a payload read on its own is a JSON Schema.
-        /// </summary>
-        public static AsyncApiMultiFormatSchema LoadMultiFormatSchemaPayload(ParseNode n)
-        {
-            return new AsyncApiMultiFormatSchema { Schema = LoadJsonSchemaPayload(n) };
-        }
-
         private static IAsyncApiSchema LoadPayload(ParseNode n, string format)
         {
             if (n == null)

src/ByteBard.AsyncAPI.Readers/V2/AsyncApiV2VersionService.cs

@@ -36,7 +36,8 @@ public AsyncApiV2VersionService(AsyncApiDiagnostic diagnostic)
             [typeof(AsyncApiAvroSchema)] = AsyncApiAvroSchemaDeserializer.LoadSchema,
             [typeof(AsyncApiJsonSchema)] = AsyncApiV2Deserializer.LoadJsonSchemaPayload,
             [typeof(AsyncApiAvroSchema)] = AsyncApiV2Deserializer.LoadAvroPayload,
-            [typeof(AsyncApiMultiFormatSchema)] = AsyncApiV2Deserializer.LoadMultiFormatSchemaPayload,
+            // 2.x has no Multi Format Schema Object. Only the local reference fallback asks for one, and it reads the document after the upgrade to 3.0.
+            [typeof(AsyncApiMultiFormatSchema)] = AsyncApiV3Deserializer.LoadMultiFormatSchema,
             [typeof(AsyncApiSecurityScheme)] = AsyncApiV2Deserializer.LoadSecurityScheme,
             [typeof(AsyncApiServer)] = AsyncApiV2Deserializer.LoadServer,
             [typeof(AsyncApiServerVariable)] = AsyncApiV2Deserializer.LoadServerVariable,

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

@@ -263,6 +263,70 @@ public void LocalPayloadReference_NotInWorkspace_ResolvesPayload(string version)
             payload.Schema.As<AsyncApiJsonSchema>().Type.Should().Be(SchemaType.String);
         }
 
+        // A local Avro payload reference that is not a registered component is resolved by falling back to the whole document.
+        [TestCase("2.6.0")]
+        [TestCase("3.0.0")]
+        public void LocalAvroPayloadReference_NotInWorkspace_ResolvesPayload(string version)
+        {
+            var yaml = version.StartsWith("2")
+                ? """
+                  asyncapi: 2.6.0
+                  info:
+                    title: test
+                    version: 1.0.0
+                  channels:
+                    workspace:
+                      publish:
+                        message:
+                          schemaFormat: application/vnd.apache.avro;version=1.9.0
+                          payload:
+                            $ref: '#/components/messages/shared/payload'
+                  components:
+                    messages:
+                      shared:
+                        schemaFormat: application/vnd.apache.avro;version=1.9.0
+                        payload:
+                          type: record
+                          name: User
+                          fields:
+                            - name: name
+                              type: string
+                  """
+                : """
+                  asyncapi: 3.0.0
+                  info:
+                    title: test
+                    version: 1.0.0
+                  channels:
+                    workspace:
+                      messages:
+                        message:
+                          payload:
+                            $ref: '#/components/messages/shared/payload'
+                  components:
+                    messages:
+                      shared:
+                        payload:
+                          schemaFormat: application/vnd.apache.avro;version=1.9.0
+                          schema:
+                            type: record
+                            name: User
+                            fields:
+                              - name: name
+                                type: string
+                  """;
+
+            var doc = new AsyncApiStringReader().Read(yaml, out var diagnostic);
+
+            diagnostic.Errors.Should().BeEmpty();
+            var payload = doc.Channels["workspace"].Messages.Values.First().Payload;
+            payload.Should().BeOfType<AsyncApiMultiFormatSchemaReference>();
+            payload.SchemaFormat.Should().StartWith("application/vnd.apache.avro");
+            var record = payload.Schema.As<AsyncApiAvroSchema>().As<AvroRecord>();
+            record.Name.Should().Be("User");
+            record.Fields.Should().ContainSingle().Which.Name.Should().Be("name");
+        }
+
         [Test]
         public void V2_ReferencePointers()
         {