feat: switch to new default model gpt-4.1

Author: azdanov

lua/CopilotChat/config.lua

@@ -61,7 +61,7 @@ return {
 
   system_prompt = 'COPILOT_INSTRUCTIONS', -- System prompt to use (can be specified manually in prompt via /).
 
-  model = 'gpt-4o', -- Default model to use, see ':CopilotChatModels' for available models (can be specified manually in prompt via $).
+  model = 'gpt-4.1', -- Default model to use, see ':CopilotChatModels' for available models (can be specified manually in prompt via $).
   agent = 'none', -- Default agent to use, see ':CopilotChatAgents' for available agents (can be specified manually in prompt via @).
   context = nil, -- Default context or array of contexts to use (can be specified manually in prompt via #).
   sticky = nil, -- Default sticky prompt or array of sticky prompts to use at start of every new chat.