fix large integers not valid

issue was: large ints in json are not valid json schema `"type":"interger"`
because `fmt.Sprintf("%v")` formats large int in exponential expression. e.g.
``` 100100100100 -> "1.001001e+11"```
Refactored to use type casting instead of `Sprintf + ParseInt`.

`validationErrorFormatNumber` mangled large integers in the same way. e.g. this is not wanted when you have DB IDs as integer in json.