define "allows the user to validate the closure of a ticket"

Users do not have much control over tickets beside opening, and updating them.

Agents do. You can also add fields that need to be populated by the agent before being closed.

I think I understand pedromrsantos 's request, as we have something similar.
Basically once the agent has submitted his/her solution to the end-user, the agent would also switch the status to "Resolved/Pending for validation". The end-user would then accept or reject the solution. At the moment I do not think there is anything out of the box for this, for a work-around would be the following:

• Agent switches to "Resolved/Pending for validation"
• Manually the end-user could write back and say yes or not with explanation
• In order to not wait indefinitely you could plug in the auto-closer: https://github.com/clonemeagain/plugin-autocloser - so that after some time with no answers it will consider the case closed and switch the status to closed.

Otherwise, one mods has been discussed here, but I haven't given it a go yet: https://forum.osticket.com/d/77062-client-closing-tickets/16