Merge pull request #1735 from ahoppen/frozen-messagekind

Mark `MessageDecodingError.MessageKind` as `@frozen`

Author: ahoppen

Sources/LanguageServerProtocol/Error.swift

@@ -137,7 +137,7 @@ public struct MessageDecodingError: Error, Hashable {
   /// If it was possible to recover the request id, it is stored here. This can be used e.g. to reply with a `ResponseError` to invalid requests.
   public var id: RequestID?
 
-  public enum MessageKind: Sendable {
+  @frozen public enum MessageKind: Sendable {
     case request
     case response
     case notification