fix: clear chat selection marks and highlights on reset

When resetting the chat, properly clean up any
remaining selection marks and highlights from the source buffer. This
prevents leftover visual indicators from persisting after the chat session
ends.

Author: deathbeam

lua/CopilotChat/init.lua

@@ -725,6 +725,14 @@ function M.stop(reset)
     state.chat:clear()
     state.last_prompt = nil
     state.last_response = nil
+
+    -- Clear the selection
+    if state.source and utils.buf_valid(state.source.bufnr) then
+      for _, mark in ipairs({ '<', '>', '[', ']' }) do
+        pcall(vim.api.nvim_buf_del_mark, state.source.bufnr, mark)
+      end
+      highlight_selection(true, state.chat.config)
+    end
   else
     client:stop()
   end