fix: use configured default_engine in chat methods

This fixes an inconsistency where chat and chat_stream methods used a
hardcoded default model ('gpt-3.5-turbo') instead of respecting the
configured default_engine like the completions method does.

Closes #15

Author: nagstler

lib/chatgpt/client.rb

@@ -31,7 +31,7 @@ def chat(messages, params = {})
       url = "#{@endpoint}/chat/completions"
 
       data = @config.default_parameters.merge(
-        model: params[:model] || 'gpt-3.5-turbo',
+        model: params[:model] || @config.default_engine, # Use configured default_engine
         messages: messages,
         temperature: params[:temperature],
         top_p: params[:top_p],
@@ -47,7 +47,7 @@ def chat_stream(messages, params = {}, &block)
 
       url = "#{@endpoint}/chat/completions"
       data = @config.default_parameters.merge(
-        model: params[:model] || 'gpt-3.5-turbo',
+        model: params[:model] || @config.default_engine,  # Use configured default_engine
         messages: messages,
         stream: true
       ).compact