style(llm): use import type for openai types in copilot provider

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dragon-fish

src/plugins/llm/providers/copilot.ts

@@ -1,4 +1,4 @@
-import { ClientOptions, OpenAI } from 'openai'
+import type { ClientOptions, OpenAI } from 'openai'
 
 import {
   ChatCompletionFeatures,