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/internal/conversion/engine_test.go

@@ -13,18 +13,20 @@ import (
 
 // mockProtocolAdapter 模拟协议适配器
 type mockProtocolAdapter struct {
-	protocolName     string
-	passthrough      bool
-	ifaceType        InterfaceType
-	supportsIface    map[InterfaceType]bool
-	decodeReqFn      func([]byte) (*canonical.CanonicalRequest, error)
-	encodeReqFn      func(*canonical.CanonicalRequest, *TargetProvider) ([]byte, error)
-	decodeRespFn     func([]byte) (*canonical.CanonicalResponse, error)
-	encodeRespFn     func(*canonical.CanonicalResponse) ([]byte, error)
-	streamDecoderFn  func() StreamDecoder
-	streamEncoderFn  func() StreamEncoder
-	rewriteReqFn     func([]byte, string, InterfaceType) ([]byte, error)
-	rewriteRespFn    func([]byte, string, InterfaceType) ([]byte, error)
+	protocolName         string
+	passthrough          bool
+	ifaceType            InterfaceType
+	supportsIface        map[InterfaceType]bool
+	decodeReqFn          func([]byte) (*canonical.CanonicalRequest, error)
+	encodeReqFn          func(*canonical.CanonicalRequest, *TargetProvider) ([]byte, error)
+	decodeRespFn         func([]byte) (*canonical.CanonicalResponse, error)
+	encodeRespFn         func(*canonical.CanonicalResponse) ([]byte, error)
+	streamDecoderFn      func() StreamDecoder
+	streamEncoderFn      func() StreamEncoder
+	rewriteReqFn         func([]byte, string, InterfaceType) ([]byte, error)
+	rewriteRespFn        func([]byte, string, InterfaceType) ([]byte, error)
+	decodeEmbeddingReqFn func([]byte) (*canonical.CanonicalEmbeddingRequest, error)
+	decodeRerankReqFn    func([]byte) (*canonical.CanonicalRerankRequest, error)
 }
 
 func newMockAdapter(name string, passthrough bool) *mockProtocolAdapter {
@@ -126,6 +128,9 @@ func (m *mockProtocolAdapter) EncodeModelInfoResponse(info *canonical.CanonicalM
 }
 
 func (m *mockProtocolAdapter) DecodeEmbeddingRequest(raw []byte) (*canonical.CanonicalEmbeddingRequest, error) {
+	if m.decodeEmbeddingReqFn != nil {
+		return m.decodeEmbeddingReqFn(raw)
+	}
 	return &canonical.CanonicalEmbeddingRequest{}, nil
 }
 
@@ -142,6 +147,9 @@ func (m *mockProtocolAdapter) EncodeEmbeddingResponse(resp *canonical.CanonicalE
 }
 
 func (m *mockProtocolAdapter) DecodeRerankRequest(raw []byte) (*canonical.CanonicalRerankRequest, error) {
+	if m.decodeRerankReqFn != nil {
+		return m.decodeRerankReqFn(raw)
+	}
 	return &canonical.CanonicalRerankRequest{}, nil
 }