Prevent callback return value from halting stream

The delta branch in decode_responses_event now explicitly returns
:ok after invoking the callback, so a callback returning {:error, _}
cannot be mistaken for a stream-level error by decode_events.

lib/open_ai/api.ex

@@ -125,6 +125,7 @@ defmodule OpenAI.API do
     case JSON.decode!(json) do
       %{"type" => "response.output_text.delta", "delta" => delta} ->
         cb.(delta)
+        :ok
 
       %{"type" => "error", "error" => %{"message" => message}} ->
         {:error, message}