add new workflow test to nightly run test suite for extensions

[aalves08]

Summary

Fixes #15849

Occurred changes and/or fixed issues

• add new workflow test to nightly run test suite for extensions

Technical notes summary

• Based on the work/testing done in test workflow test for create-pr-build-extension-charts #15893, where we've had a successful workflow run, as per https://github.com/rancher/dashboard/actions/runs/19175210519

Here's proof of correct output for the test above in Slack:

NOTE: Bear in mind that the only side-effect here is that we're creating a PR in rancher/dashboard, which does get closed automatically in the end of the workflow job, along with clearing out the branch that is also created

Areas or cases that should be tested

Areas which could experience regressions

Screenshot/Video

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
• The PR has been reviewed in terms of Accessibility
• The PR has considered, and if applicable tested with, the three Global Roles Admin, Standard User and User Base

[cnotv]

LGTM