fix: correct chat config handling

When opening chat and asking questions, ensure proper config inheritance
by deep extending chat state config with provided config and global config.
Also move config handling before early returns to maintain consistency.

Closes #676

lua/CopilotChat/init.lua

@@ -599,6 +599,10 @@ function M.open(config)
   end
 
   config = vim.tbl_deep_extend('force', M.config, config or {})
+  if config.headless then
+    return
+  end
+
   utils.return_to_normal_mode()
   state.chat:open(config)
   state.chat:follow()
@@ -676,18 +680,18 @@ end
 ---@param prompt string?
 ---@param config CopilotChat.config.shared?
 function M.ask(prompt, config)
-  config = vim.tbl_deep_extend('force', M.config, config or {})
-  vim.diagnostic.reset(vim.api.nvim_create_namespace('copilot_diagnostics'))
+  M.open(config)
 
-  if not config.headless then
-    M.open(config)
-  end
+  config = vim.tbl_deep_extend('force', state.chat.config, config or {})
+  config = vim.tbl_deep_extend('force', M.config, config or {})
 
   prompt = vim.trim(prompt or '')
   if prompt == '' then
     return
   end
 
+  vim.diagnostic.reset(vim.api.nvim_create_namespace('copilot_diagnostics'))
+
   if not config.headless then
     if config.clear_chat_on_new_prompt then
       M.stop(true)