Generate secure URL safe token
Minor suggestions:

public class UrlToken


The class has no instance data, so it could be made static:

public static class UrlToken


Microsoft's Naming Guidelines and their Framework Design Guidelines suggest not using underscores and also using PascalCasing for constants, so

    private const int BYTE_LENGTH = 32; 


could be:

    private const int ByteLength = 32; 


However, even that name doesn't tell us much of what it is for. Let's try again:

    private const int NumberOfRandomBytes = 32; 


Typo/misspelling in the XML doc comment: "encoding" is written as "endcoding".

There is mixed curly brace formatting. Microsoft guidelines (see links above) suggest the opening and closing curly braces should be on their own line.

        using (RNGCryptoServiceProvider provider = new RNGCryptoServiceProvider()) { 


to:

        using (RNGCryptoServiceProvider provider = new RNGCryptoServiceProvider())
        { 


By the way, kudos to you on your proper use of the using construct! Looks fantastic!