fix: inline a 2.x channel reference whose target is not written

A 2.x channel reference is written with its target's address, since 2.x
keys channels by address. When the target is not one of the document's
channels, for example because it was removed after reading, that address
points nowhere and did not match the reference, so it was left dangling.
Such a reference now keeps its own pointer, which is found dangling and
written inline.

Author: VisualBean

src/ByteBard.AsyncAPI/Models/References/AsyncApiChannelReference.cs

@@ -3,6 +3,7 @@ namespace ByteBard.AsyncAPI.Models
     using System;
     using System.Collections.Generic;
     using System.Diagnostics;
+    using System.Linq;
     using ByteBard.AsyncAPI.Models.Interfaces;
     using ByteBard.AsyncAPI.Writers;
 
@@ -56,9 +57,12 @@ public override void SerializeV2(IAsyncApiWriter writer)
             if (!writer.GetSettings().ShouldInlineReference(this.Reference))
             {
                 this.Reference.Workspace = writer.Workspace;
-                if (this.IsRootChannelReference() && this.Target?.Address != null)
+                if (this.IsRootChannelReference()
+                    && this.Target?.Address != null
+                    && writer.Workspace.RootDocument?.Channels.Values.Any(channel => ReferenceEquals(channel, this.Target)) == true)
                 {
-                    // 2.x keys root channels by address instead of by key.
+                    // 2.x keys root channels by address instead of by key. A target the document does not write
+                    // keeps its own reference, so it can be found dangling and written inline.
                     writer.WriteStartObject();
                     writer.WriteOptionalProperty(AsyncApiConstants.DollarRef, "#/channels/" + JsonPointer.EscapeToken(this.Target.Address));
                     writer.WriteEndObject();

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

@@ -591,6 +591,19 @@ public void WriteAsV2_ReadsBackTheSame(string name)
             this.AssertRoundTrip(name, AsyncApiVersion.AsyncApi2_0);
         }
 
+        [Test]
+        public void WriteAsV2_ChannelReference_ToARemovedChannel_IsInlined()
+        {
+            var document = Read(Cases["v2 channel by address"].Yaml);
+
+            // The reference still resolves to the channel it was read with, but the document no longer has it.
+            document.Channels.Remove("usersignedup");
+            var written = document.SerializeAsYaml(AsyncApiVersion.AsyncApi2_0);
+
+            References(document.SerializeAsJson(AsyncApiVersion.AsyncApi2_0)).Should().NotContain(r => r.StartsWith("#/channels/"), written);
+            Cases["v2 channel by address"].Inspect(Read(written)).Should().Be(Marker, written);
+        }
+
         [TestCaseSource(nameof(CaseNames))]
         public void WriteAsV3_ReadsBackTheSame(string name)
         {