fix: filter GPT-5.2 Pro from chat (Responses API only, not chat)

GPT-5.2 Pro and its dated variant only support the Responses API,
not Chat Completions. OCI GenAI uses chat/completions internally,
causing a 400 error. Also trim debug log to a shorter format.

backend/app.py

@@ -919,6 +919,7 @@ def _call_genai(gc: dict, message: str, history: list = None) -> str:
         region = gc.get("genai_region", "")
         ocids = model_info.get("ocids", {})
         model_ref = ocids.get(region) or gc["model_id"]
+    log.info(f"GenAI call: model_id={gc.get('model_id')}, model_ref={model_ref[:60]}...")
     if gc.get("serving_type") == "DEDICATED" and gc.get("dedicated_endpoint_id"):
         chat_detail.serving_mode = oci.generative_ai_inference.models.DedicatedServingMode(
             endpoint_id=gc["dedicated_endpoint_id"]