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/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"