refactor: 将 OpType 字段从枚举类型改为 string 类型

主要变更：
- Operation.OpType: Type → string
- NewFullOperation 参数: opType Type → opType string
- IsValidOpType 参数: opType Type → opType string
- operationMeta.OpType: *Type → *string
- queryclient.ListRequest.OpType: model.Type → string

优点：
- 更灵活，支持动态扩展操作类型
- 不再受限于预定义的枚举常量
- 简化类型转换逻辑

兼容性：
- Type 常量定义保持不变 (OpTypeCreate, OpTypeUpdate 等)
- 使用时需要 string() 转换: string(model.OpTypeCreate)
- 所有单元测试已更新并通过 (100%)

测试结果：
✅ api/adapter - PASS
✅ api/highclient - PASS
✅ api/logger - PASS
✅ api/model - PASS
✅ api/persistence - PASS
✅ api/queryclient - PASS
✅ internal/* - PASS

api/model/envelope_test.go

@@ -196,7 +196,7 @@ func TestMarshalTrustlog_Basic(t *testing.T) {
 		OpID:         "op-123",
 		Timestamp:    time.Now(),
 		OpSource:     model.OpSourceIRP,
-		OpType:       model.OpTypeOCCreateHandle,
+		OpType:       string(model.OpTypeOCCreateHandle),
 		DoPrefix:     "test",
 		DoRepository: "repo",
 		Doid:         "test/repo/123",
@@ -229,7 +229,7 @@ func TestMarshalOperation(t *testing.T) {
 		OpID:         "op-123",
 		Timestamp:    time.Now(),
 		OpSource:     model.OpSourceIRP,
-		OpType:       model.OpTypeOCCreateHandle,
+		OpType:       string(model.OpTypeOCCreateHandle),
 		DoPrefix:     "test",
 		DoRepository: "repo",
 		Doid:         "test/repo/123",
@@ -253,7 +253,7 @@ func TestUnmarshalOperation(t *testing.T) {
 		OpID:         "op-123",
 		Timestamp:    time.Now(),
 		OpSource:     model.OpSourceIRP,
-		OpType:       model.OpTypeOCCreateHandle,
+		OpType:       string(model.OpTypeOCCreateHandle),
 		DoPrefix:     "test",
 		DoRepository: "repo",
 		Doid:         "test/repo/123",