fix: Fix json index does not work for string filter (#41383)

pr: #41382 
issue: #35528

Signed-off-by: sunby <sunbingyi1992@gmail.com>

Author: sunby

internal/core/src/common/Types.h

@@ -580,7 +580,7 @@ FromValCase(milvus::proto::plan::GenericValue::ValCase val_case) {
         case milvus::proto::plan::GenericValue::ValCase::kFloatVal:
             return DataType::DOUBLE;
         case milvus::proto::plan::GenericValue::ValCase::kStringVal:
-            return DataType::STRING;
+            return DataType::VARCHAR;
         case milvus::proto::plan::GenericValue::ValCase::kArrayVal:
             return DataType::ARRAY;
         default: