fix: read multi-format payload fallbacks in 2.x documents

ResolveStreamReference is also the fallback for local references that are
not in the workspace, not only for external ones. In a 2.x document every
local payload reference is an AsyncApiMultiFormatSchemaReference, so the
fallback asked the 2.x reader for an AsyncApiMultiFormatSchema loader it
did not have and Read() threw KeyNotFoundException.

The 2.x reader now loads a Multi Format Schema by reading the payload as a
JSON Schema and wrapping it, since 2.x keeps schemaFormat on the message.

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

Author: bielu

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

@@ -73,6 +73,14 @@ 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

@@ -36,6 +36,7 @@ public AsyncApiV2VersionService(AsyncApiDiagnostic diagnostic)
             [typeof(AsyncApiAvroSchema)] = AsyncApiAvroSchemaDeserializer.LoadSchema,
             [typeof(AsyncApiJsonSchema)] = AsyncApiV2Deserializer.LoadJsonSchemaPayload,
             [typeof(AsyncApiAvroSchema)] = AsyncApiV2Deserializer.LoadAvroPayload,
+            [typeof(AsyncApiMultiFormatSchema)] = AsyncApiV2Deserializer.LoadMultiFormatSchemaPayload,
             [typeof(AsyncApiSecurityScheme)] = AsyncApiV2Deserializer.LoadSecurityScheme,
             [typeof(AsyncApiServer)] = AsyncApiV2Deserializer.LoadServer,
             [typeof(AsyncApiServerVariable)] = AsyncApiV2Deserializer.LoadServerVariable,

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

@@ -214,6 +214,55 @@ public void V3_ExternalPayloadReference_WithoutFragment_ResolvesPayload()
             doc.Channels["workspace"].Messages["message"].Payload.Schema.As<AsyncApiJsonSchema>().Type.Should().Be(SchemaType.Boolean);
         }
 
+        // A local 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 LocalPayloadReference_NotInWorkspace_ResolvesPayload(string version)
+        {
+            var yaml = version.StartsWith("2")
+                ? """
+                  asyncapi: 2.6.0
+                  info:
+                    title: test
+                    version: 1.0.0
+                  channels:
+                    workspace:
+                      publish:
+                        message:
+                          payload:
+                            $ref: '#/components/messages/shared/payload'
+                  components:
+                    messages:
+                      shared:
+                        payload:
+                          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:
+                          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.Schema.As<AsyncApiJsonSchema>().Type.Should().Be(SchemaType.String);
+        }
+
         [Test]
         public void V2_ReferencePointers()
         {