Fix: NameError when instantiating MiniMax client

Resolve name error where OpenAI was not defined in the scope of _ensure_minimax_client by calling openai.OpenAI directly.

src/ai_client.py

@@ -2133,7 +2133,7 @@ def _ensure_minimax_client() -> None:
   api_key = creds.get("minimax", {}).get("api_key")
   if not api_key:
    raise ValueError("MiniMax API key not found in credentials.toml")
-  _minimax_client = OpenAI(api_key=api_key, base_url="https://api.minimax.chat/v1")
+  _minimax_client = openai.OpenAI(api_key=api_key, base_url="https://api.minimax.chat/v1")
 
 def _ensure_grok_client() -> Any:
  global _grok_client