Additional validation rules for multi-select checkboxes

## User story

As a form builder
I want to specify the number of checkboxes to select from a list
So that I can ask for a certain number of options only

## Context

Some users have asked for options to decide whether there is a minimum or maximum number of answers to be checked when using multi-select choice questions.

https://cds-snc.freshdesk.com/a/tickets/29353
> How do I set up a checkbox question to only allow three responses?

https://gcdigital.slack.com/archives/C09CM76GCS0/p1788274400365599
> Il serait utile que GC Forms offre un format de question permettant aux répondants de sélectionner un nombre maximal de réponses, par exemple : Sélectionnez jusqu’à 3 options parmi les choix suivants.

https://gcdigital.slack.com/archives/C09CM76GCS0/p1788288311629909
> Limit multiple choice responses (eg select top 5 from a list) would be useful. Built in analysis / dashboard would be amazing

No files, tests, or entry points are named in the issue. Start by locating the multi-select choice question and its validation path, then clarify whether minimum, maximum, or exact selection counts are required. Done means form builders can configure these limits and respondents cannot submit invalid selections, with tests covering the validation behavior.

Written by the indexing model from the issue text.