Default to gpt-4o-mini instead of gpt-3.5-turbo in tests, func args

GPT-4o-mini is cheaper, smarter and can hold more context than
GPT-3.5-turbo. In production, we also default to gpt-4o-mini, so makes
sense to upgrade defaults and tests to work with it

src/khoj/processor/conversation/openai/gpt.py

@@ -112,7 +112,7 @@ def converse(
     user_query,
     online_results: Optional[Dict[str, Dict]] = None,
     conversation_log={},
-    model: str = "gpt-3.5-turbo",
+    model: str = "gpt-4o-mini",
     api_key: Optional[str] = None,
     api_base_url: Optional[str] = None,
     temperature: float = 0.2,