feat: 系统性改进后端测试体系

- 新增 6 个测试场景 (config load pipe, handler errors, service aggregation, engine degradation, openai decoder edges, negative tests)
- 更新测试工具规格 (mockgen, in-memory SQLite)
- 覆盖率目标从 >80% 提升至 >85%
- 新增 test-unit 和 test-integration Makefile 命令
- 新增死代码清理和 mockgen 需求
- 归档变更至 openspec/changes/archive/2026-04-22-improve-backend-testing/

backend/tests/mocks/mock_provider_client.go

@@ -0,0 +1,73 @@
+// Code generated by MockGen. DO NOT EDIT.
+// Source: client.go
+//
+// Generated by this command:
+//
+//	mockgen -source=client.go -destination=../../tests/mocks/mock_provider_client.go -package=mocks
+//
+
+// Package mocks is a generated GoMock package.
+package mocks
+
+import (
+	context "context"
+	conversion "nex/backend/internal/conversion"
+	provider "nex/backend/internal/provider"
+	reflect "reflect"
+
+	gomock "go.uber.org/mock/gomock"
+)
+
+// MockProviderClient is a mock of ProviderClient interface.
+type MockProviderClient struct {
+	ctrl     *gomock.Controller
+	recorder *MockProviderClientMockRecorder
+	isgomock struct{}
+}
+
+// MockProviderClientMockRecorder is the mock recorder for MockProviderClient.
+type MockProviderClientMockRecorder struct {
+	mock *MockProviderClient
+}
+
+// NewMockProviderClient creates a new mock instance.
+func NewMockProviderClient(ctrl *gomock.Controller) *MockProviderClient {
+	mock := &MockProviderClient{ctrl: ctrl}
+	mock.recorder = &MockProviderClientMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use.
+func (m *MockProviderClient) EXPECT() *MockProviderClientMockRecorder {
+	return m.recorder
+}
+
+// Send mocks base method.
+func (m *MockProviderClient) Send(ctx context.Context, spec conversion.HTTPRequestSpec) (*conversion.HTTPResponseSpec, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "Send", ctx, spec)
+	ret0, _ := ret[0].(*conversion.HTTPResponseSpec)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// Send indicates an expected call of Send.
+func (mr *MockProviderClientMockRecorder) Send(ctx, spec any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Send", reflect.TypeOf((*MockProviderClient)(nil).Send), ctx, spec)
+}
+
+// SendStream mocks base method.
+func (m *MockProviderClient) SendStream(ctx context.Context, spec conversion.HTTPRequestSpec) (<-chan provider.StreamEvent, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "SendStream", ctx, spec)
+	ret0, _ := ret[0].(<-chan provider.StreamEvent)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// SendStream indicates an expected call of SendStream.
+func (mr *MockProviderClientMockRecorder) SendStream(ctx, spec any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "SendStream", reflect.TypeOf((*MockProviderClient)(nil).SendStream), ctx, spec)
+}