fix(ai_client): Use send_message_stream for google-genai streaming

src/ai_client.py

@@ -685,7 +685,7 @@ def _send_gemini(md_content: str, user_message: str, base_dir: str,
      temperature=_temperature,
      max_output_tokens=_max_tokens,
     )
-    resp = _gemini_chat.send_message(payload, config=config, stream=True)
+    resp = _gemini_chat.send_message_stream(payload, config=config)
     txt_chunks: list[str] = []
     calls = []
     usage = {}