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/converter_test.go

@@ -54,7 +54,7 @@ func TestFromProtobuf_Basic(t *testing.T) {
 	assert.Equal(t, "op-123", result.OpID)
 	assert.Equal(t, now.Unix(), result.Timestamp.Unix())
 	assert.Equal(t, model.Source("IRP"), result.OpSource)
-	assert.Equal(t, model.Type("OC_CREATE_HANDLE"), result.OpType)
+	assert.Equal(t, "OC_CREATE_HANDLE", result.OpType)
 	assert.Equal(t, "test", result.DoPrefix)
 	assert.Equal(t, "repo", result.DoRepository)
 	assert.Equal(t, "test/repo/123", result.Doid)
@@ -133,7 +133,7 @@ func TestToProtobuf_Basic(t *testing.T) {
 		OpID:         "op-123",
 		Timestamp:    now,
 		OpSource:     model.OpSourceIRP,
-		OpType:       model.OpTypeOCCreateHandle,
+		OpType:       string(model.OpTypeOCCreateHandle),
 		DoPrefix:     "test",
 		DoRepository: "repo",
 		Doid:         "test/repo/123",
@@ -166,7 +166,7 @@ func TestToProtobuf_WithHashes(t *testing.T) {
 		OpID:             "op-123",
 		Timestamp:        now,
 		OpSource:         model.OpSourceDOIP,
-		OpType:           model.OpTypeCreate,
+		OpType:       string(model.OpTypeCreate),
 		DoPrefix:         "test",
 		DoRepository:     "repo",
 		Doid:             "test/repo/123",
@@ -192,7 +192,7 @@ func TestToProtobuf_WithoutHashes(t *testing.T) {
 		OpID:         "op-123",
 		Timestamp:    now,
 		OpSource:     model.OpSourceDOIP,
-		OpType:       model.OpTypeCreate,
+		OpType:       string(model.OpTypeCreate),
 		DoPrefix:     "test",
 		DoRepository: "repo",
 		Doid:         "test/repo/123",
@@ -511,7 +511,7 @@ func TestRoundTrip_Operation(t *testing.T) {
 		OpID:         "op-123",
 		Timestamp:    now,
 		OpSource:     model.OpSourceIRP,
-		OpType:       model.OpTypeOCCreateHandle,
+		OpType:       string(model.OpTypeOCCreateHandle),
 		DoPrefix:     "test",
 		DoRepository: "repo",
 		Doid:         "test/repo/123",