feat: add fallback chat model chain in tool loop runner (#5109)

* feat: implement fallback provider support for chat models and update configuration

* feat: enhance provider selection display with count and chips for selected providers

* feat: update fallback chat providers to use provider settings and add warning for non-list fallback models

tests/test_tool_loop_agent_runner.py

@@ -90,6 +90,21 @@ class MockToolExecutor:
         return generator()
 
 
+class MockFailingProvider(MockProvider):
+    async def text_chat(self, **kwargs) -> LLMResponse:
+        self.call_count += 1
+        raise RuntimeError("primary provider failed")
+
+
+class MockErrProvider(MockProvider):
+    async def text_chat(self, **kwargs) -> LLMResponse:
+        self.call_count += 1
+        return LLMResponse(
+            role="err",
+            completion_text="primary provider returned error",
+        )
+
+
 class MockHooks(BaseAgentRunHooks):
     """模拟钩子函数"""
 
@@ -321,6 +336,64 @@ async def test_hooks_called_with_max_step(
     assert mock_hooks.tool_end_called, "on_tool_end应该被调用"
 
 
+@pytest.mark.asyncio
+async def test_fallback_provider_used_when_primary_raises(
+    runner, provider_request, mock_tool_executor, mock_hooks
+):
+    primary_provider = MockFailingProvider()
+    fallback_provider = MockProvider()
+    fallback_provider.should_call_tools = False
+
+    await runner.reset(
+        provider=primary_provider,
+        request=provider_request,
+        run_context=ContextWrapper(context=None),
+        tool_executor=mock_tool_executor,
+        agent_hooks=mock_hooks,
+        streaming=False,
+        fallback_providers=[fallback_provider],
+    )
+
+    async for _ in runner.step_until_done(5):
+        pass
+
+    final_resp = runner.get_final_llm_resp()
+    assert final_resp is not None
+    assert final_resp.role == "assistant"
+    assert final_resp.completion_text == "这是我的最终回答"
+    assert primary_provider.call_count == 1
+    assert fallback_provider.call_count == 1
+
+
+@pytest.mark.asyncio
+async def test_fallback_provider_used_when_primary_returns_err(
+    runner, provider_request, mock_tool_executor, mock_hooks
+):
+    primary_provider = MockErrProvider()
+    fallback_provider = MockProvider()
+    fallback_provider.should_call_tools = False
+
+    await runner.reset(
+        provider=primary_provider,
+        request=provider_request,
+        run_context=ContextWrapper(context=None),
+        tool_executor=mock_tool_executor,
+        agent_hooks=mock_hooks,
+        streaming=False,
+        fallback_providers=[fallback_provider],
+    )
+
+    async for _ in runner.step_until_done(5):
+        pass
+
+    final_resp = runner.get_final_llm_resp()
+    assert final_resp is not None
+    assert final_resp.role == "assistant"
+    assert final_resp.completion_text == "这是我的最终回答"
+    assert primary_provider.call_count == 1
+    assert fallback_provider.call_count == 1
+
+
 if __name__ == "__main__":
     # 运行测试
     pytest.main([__file__, "-v"])