add node pool name length validation and refactor to allow unit tests

[mantis-toboggan-md]

Summary

Fixes #11405 - see comments for expected behavior

Occurred changes and/or fixed issues

This PR updates the node pool name validator to check length as well as characters.

Areas or cases that should be tested

Open the AKS provisioning form and enter a pool name with >12 characters

1. You should be prevented from saving
2. There should be an error icon in the node pool name input
3. If you add another pool and switch pool tabs, the inactive tab with validation error should have an error icon

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
• The PR has a Milestone
• The PR template has been filled out
• The PR has been self reviewed
• The PR has a reviewer assigned
• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes