fix: 修正 conversion 代理路径和错误边界

backend/tests/integration/conversion_test.go

@@ -521,15 +521,14 @@ func TestConversion_OldRoutes_Return404(t *testing.T) {
 	req := httptest.NewRequest("POST", "/v1/chat/completions", strings.NewReader(`{"model":"test"}`))
 	req.Header.Set("Content-Type", "application/json")
 	r.ServeHTTP(w, req)
-	// Gin 路由匹配但协议不支持返回 400
-	assert.Equal(t, 400, w.Code)
+	assert.Equal(t, 404, w.Code)
 
 	// 旧 Anthropic 路由
 	w = httptest.NewRecorder()
 	req = httptest.NewRequest("POST", "/v1/messages", strings.NewReader(`{"model":"test"}`))
 	req.Header.Set("Content-Type", "application/json")
 	r.ServeHTTP(w, req)
-	assert.Equal(t, 400, w.Code)
+	assert.Equal(t, 404, w.Code)
 }
 
 // ============ Provider Protocol 字段测试 ============

backend/tests/mocks/mock_provider_client.go

@@ -59,10 +59,10 @@ func (mr *MockProviderClientMockRecorder) Send(ctx, spec any) *gomock.Call {
 }
 
 // SendStream mocks base method.
-func (m *MockProviderClient) SendStream(ctx context.Context, spec conversion.HTTPRequestSpec) (<-chan provider.StreamEvent, error) {
+func (m *MockProviderClient) SendStream(ctx context.Context, spec conversion.HTTPRequestSpec) (*provider.StreamResponse, error) {
 	m.ctrl.T.Helper()
 	ret := m.ctrl.Call(m, "SendStream", ctx, spec)
-	ret0, _ := ret[0].(<-chan provider.StreamEvent)
+	ret0, _ := ret[0].(*provider.StreamResponse)
 	ret1, _ := ret[1].(error)
 	return ret0, ret1
 }