Add async tests for chat handler and context fallback handling

backend/tests/test_agent_chat_tool.py

@@ -1,7 +1,9 @@
+import asyncio
 import unittest
 from types import SimpleNamespace
+from unittest.mock import patch
 
-from backend.agent.tools.chat import _drop_superseded_trailing_user_rows
+from backend.agent.tools.chat import _drop_superseded_trailing_user_rows, chat_handler
 
 
 class ChatToolTests(unittest.TestCase):
@@ -26,3 +28,44 @@ class ChatToolTests(unittest.TestCase):
 
         self.assertIs(_drop_superseded_trailing_user_rows(rows), rows)
 
+
+class ChatToolAsyncTests(unittest.IsolatedAsyncioTestCase):
+    async def test_chat_handler_returns_context_fallback_when_model_fails(self):
+        async def emit(_event_type, _payload):
+            return None
+
+        async def failing_stream(*_args, **_kwargs):
+            raise RuntimeError("model returned 502")
+            if False:
+                yield None
+
+        context = SimpleNamespace(
+            session_id=None,
+            db_factory=lambda: None,
+            emit=emit,
+            cancellation_event=asyncio.Event(),
+            show_thinking=False,
+        )
+        arguments = {
+            "model": "broken-model",
+            "messages": [{"role": "user", "content": "weather?"}],
+            "context_blocks": [{
+                "context_block": "<websearch_context>Weather context</websearch_context>",
+                "sources": [{
+                    "type": "web",
+                    "title": "Schwäbisch Gmünd Weather",
+                    "snippet": "Sunny intervals and light wind.",
+                    "url": "https://weather.example/schwaebisch-gmuend",
+                }],
+            }],
+            "generation_options": {},
+            "reasoning": False,
+        }
+
+        with patch("backend.agent.tools.chat.chat_stream_typed", failing_stream):
+            result = await chat_handler(arguments, context)
+
+        self.assertIn("selected chat model failed", result["content"])
+        self.assertIn("Schwäbisch Gmünd Weather", result["content"])
+        self.assertIn("https://weather.example/schwaebisch-gmuend", result["content"])
+        self.assertEqual(result["sources"][0]["url"], "https://weather.example/schwaebisch-gmuend")