fix(chat): release Utf8String when linkshell check rejects channel

Cherry-pick from ChatTwo upstream f35b7d3 (Infiziert90, 2026-05-12).

Chat.SetChannel allocates a native Utf8String for the target name and
then runs a validity check. The previous early return on an invalid
linkshell skipped Dtor and leaked the native allocation; every invalid
linkshell switch added one Utf8String to the unmanaged heap.

- Renamed ValidAnyLinkshell to IsChannelOrExistingLinkshell so the
  call-site reads naturally.
- Wrapped ChangeChatChannel in the validity check instead of
  early-returning. Dtor now runs on every path.
- ChatLogWindow follows the rename at its single call-site.

HellionChat/GameFunctions/Chat.cs

@@ -423,16 +423,24 @@ internal sealed unsafe class Chat : IDisposable
         );
     }
 
-    // Check if channel is valid (non-linkshell or existing linkshell)
-    internal static bool ValidAnyLinkshell(InputChannel channel)
+    // ---------------------------------------------------------------
+    // Cherry-picked from ChatTwo upstream f35b7d3 (Infiziert90, 2026-05-12)
+    // - Renamed ValidAnyLinkshell -> IsChannelOrExistingLinkshell. The
+    //   name now states intent: returns true for any non-linkshell
+    //   channel, or a linkshell index that actually exists.
+    // ---------------------------------------------------------------
+    internal static bool IsChannelOrExistingLinkshell(InputChannel channel)
     {
         var idx = channel.LinkshellIndex();
         if (idx == uint.MaxValue || channel.IsExtraChatLinkshell())
             return true;
-        if (channel.IsLinkshell() && ValidLinkshell(idx))
-            return true;
-        if (channel.IsCrossLinkshell() && ValidCrossLinkshell(idx))
-            return true;
+
+        if (channel.IsLinkshell())
+            return ValidLinkshell(idx);
+
+        if (channel.IsCrossLinkshell())
+            return ValidCrossLinkshell(idx);
+
         return false;
     }
 
@@ -531,12 +539,17 @@ internal sealed unsafe class Chat : IDisposable
         if (idx == uint.MaxValue)
             idx = 0;
 
-        if (!ValidAnyLinkshell(channel))
-            return;
-
+        // ---------------------------------------------------------------
+        // Cherry-picked from ChatTwo upstream f35b7d3 (Infiziert90, 2026-05-12)
+        // - Wrap ChangeChatChannel in the validity check instead of
+        //   early-returning. The previous early return skipped Dtor and
+        //   leaked the native Utf8String allocated a few lines above.
+        // ---------------------------------------------------------------
+        if (IsChannelOrExistingLinkshell(channel))
             RaptureShellModule
                 .Instance()
                 ->ChangeChatChannel(tellTarget != null ? 17 : (int)channel, idx, target, true);
+
         target->Dtor(true);
     }
 

HellionChat/Ui/ChatLogWindow.cs

@@ -272,7 +272,10 @@ public sealed class ChatLogWindow : Window
                 }
             }
 
-            if (targetChannel == null || !GameFunctions.Chat.ValidAnyLinkshell(targetChannel.Value))
+            if (
+                targetChannel == null
+                || !GameFunctions.Chat.IsChannelOrExistingLinkshell(targetChannel.Value)
+            )
             {
                 Plugin.Log.Warning(
                     $"Channel was set to an invalid value '{targetChannel}', ignoring"