fix: notify server when model is selected via EcaChatSelectModel

When a model was selected via EcaChatSelectModel, the local state was
updated but the server was never notified. This meant the server's
last-config-notified was out of sync with the client's selected model.

Send chat/selectedModelChanged to the server on model selection,
matching the approach used by eca-emacs. The local state update is
kept for immediate header refresh since the server's response to
chat/selectedModelChanged does not include selectModel.

Note: /model in chat does not update the header when the selected model
matches the server's last-config-notified value. This is a server-side
gap where chat-selected-model-changed should include select-model in its
notify-fields-changed-only! call to keep last-config-notified in sync.

Author: ssjoleary

lua/eca/commands.lua

@@ -454,6 +454,7 @@ function M.setup()
     }, function(choice)
       if choice then
         chat.mediator:update_selected_model(choice)
+        chat.mediator:send("chat/selectedModelChanged", { model = choice, variant = nil }, nil)
       end
     end)
   end, {