fix: write 2.x channel servers as server names

AsyncAPI 2.x lists a channel's servers by name, but the 2.x writer emitted
the reference fragment, so "#/servers/production" was written as
"/servers/production". This also broke a 2.x read/write round trip.

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

Author: bielu

src/ByteBard.AsyncAPI/Models/AsyncApiChannel.cs

@@ -84,7 +84,7 @@ public virtual void SerializeV2(IAsyncApiWriter writer)
                 writer.WriteOptionalProperty(AsyncApiConstants.Description, this.Description);
 
                 // servers
-                writer.WriteOptionalCollection(AsyncApiConstants.Servers, this.Servers.Select(s => s.Reference.FragmentId).ToList(), (w, s) => w.WriteValue(s));
+                writer.WriteOptionalCollection(AsyncApiConstants.Servers, this.Servers.Select(GetV2ServerName).ToList(), (w, s) => w.WriteValue(s));
 
                 var operations = writer.Workspace.RootDocument?.Operations.Values.Where(operation => CheckOperationChannel(operation, writer)).ToList();
 
@@ -166,5 +166,20 @@ private bool CheckOperationChannel(AsyncApiOperation operation, IAsyncApiWriter
             operation.Channel.Reference.Workspace = writer.Workspace;
             return operation.Channel.Equals(this);
         }
+
+        /// <summary>
+        /// AsyncAPI 2.x lists channel servers by name, so a "#/servers/{name}" pointer is written as "{name}".
+        /// </summary>
+        private static string GetV2ServerName(AsyncApiServerReference server)
+        {
+            var fragmentId = server.Reference.FragmentId;
+            if (fragmentId == null)
+            {
+                return server.Reference.Reference;
+            }
+
+            var tokens = new JsonPointer(fragmentId).Tokens;
+            return tokens.Length == 2 && tokens[0] == AsyncApiConstants.Servers ? tokens[1] : fragmentId;
+        }
     }
 }

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

@@ -156,6 +156,53 @@ public void V2_AsyncApiChannel_UpgradesAndNormalizesKey()
             reserialized.Should().BePlatformAgnosticEquivalentTo(yaml);
         }
 
+        [Test]
+        public void V2_AsyncApiChannel_WithServers_SerializesServerNames()
+        {
+            var yaml =
+                """
+                asyncapi: 2.6.0
+                info:
+                  title: test spec
+                  version: 1.0.0
+                servers:
+                  production:
+                    url: example.com
+                    protocol: kafka
+                channels:
+                  mychannel:
+                    servers:
+                      - production
+                """;
+
+            var document = new AsyncApiStringReader().Read(yaml, out var diag);
+
+            document.Channels.First().Value.Servers.Single().Reference.Reference.Should().Be("#/servers/production");
+
+            var reserialized = document.SerializeAsYaml(AsyncApiVersion.AsyncApi2_0);
+
+            reserialized.Should().BePlatformAgnosticEquivalentTo(yaml);
+        }
+
+        [Test]
+        public void V2_AsyncApiChannel_WithServerReference_SerializesServerName()
+        {
+            var expected =
+                """
+                servers:
+                  - production
+                """;
+
+            var channel = new AsyncApiChannel
+            {
+                Servers = new List<AsyncApiServerReference> { new AsyncApiServerReference("#/servers/production") },
+            };
+
+            var actual = channel.SerializeAsYaml(AsyncApiVersion.AsyncApi2_0);
+
+            actual.Should().BePlatformAgnosticEquivalentTo(expected);
+        }
+
         [Test]
         public void V2_AsyncApiChannel_reserializes()
         {