fix: Use model backend when creating BackendParams

Assign the backend from the chat model before creating BackendParams and use it to initialize BackendParams and to set message types. This ensures message Type mapping is based on model.Backend (and avoids relying on BackendParams.Backend prior to initialization), clarifying initialization order and avoiding potential mismatches.

Author: Madzionator

src/MaIN.Services/Services/ChatService.cs

@@ -44,8 +44,8 @@ public async Task<ChatResult> Completions(
             throw new ChatModelNotAvailableException(chat.Id, chat.ModelId);
         }
 
-        chat.BackendParams ??= BackendParamsFactory.Create(model!.Backend);
-        var backend = chat.BackendParams.Backend;
+        var backend = model!.Backend;
+        chat.BackendParams ??= BackendParamsFactory.Create(backend);
 
         chat.Messages.Where(x => x.Type == MessageType.NotSet).ToList()
             .ForEach(x => x.Type = backend != BackendType.Self ? MessageType.CloudLLM : MessageType.LocalLLM);