Fix varchar cast for json

Summary:
https://www.internalfb.com/tasks/?t=211442303

There was an error in running query on Prestissimo not Presto - "Scalar function presto.default.substr not registered with arguments: (JSON, BIGINT, BIGINT)".

This is not due to missing function, as the function signature does not exist in Presto. It occurs when attempting to cast JSON as varchar of capped length.
Related Diff: https://www.internalfb.com/diff/D59531026
Note: Exception is still raised for try_cast() behavior. Alignment is out of scope for this PR

Differential Revision: D68353517

presto-native-execution/presto_cpp/main/types/PrestoToVeloxExpr.cpp

@@ -181,8 +181,15 @@ std::optional<TypedExprPtr> convertCastToVarcharWithMaxLength(
   VELOX_DCHECK(end == returnType.data() + returnType.size() - 1);
 
   VELOX_DCHECK_EQ(args.size(), 1);
-  const auto arg = args[0];
 
+  auto arg = args[0];
+  // If the argument is of JSON type, convert it to VARCHAR before applying substr.
+  if (velox::isJsonType(arg->type())) {
+    arg = std::make_shared<CallTypedExpr>(
+        velox::VARCHAR(),
+        std::vector<TypedExprPtr>{arg},
+        "presto.default.json_format");
+  }
   return std::make_shared<CallTypedExpr>(
       arg->type(),
       std::vector<TypedExprPtr>{