Use Responses API with Luna by default

backend/tests/test_agent_config.py

@@ -0,0 +1,14 @@
+"""Assistant transport configuration."""
+
+from agent_framework.openai import OpenAIChatClient
+
+from app.agent import agent as agent_module
+
+
+def test_agent_uses_responses_client_for_the_configured_model(monkeypatch):
+    monkeypatch.setattr(agent_module.settings, "AGENT_MODEL", "gpt-5.6-luna")
+
+    agent = agent_module.build_agent()
+
+    assert isinstance(agent.client, OpenAIChatClient)
+    assert agent.client.model == "gpt-5.6-luna"