Prerequisites checklist

• I have read the contributing guidelines.

AI acknowledgment

• I did not use AI to generate this PR.
• (If the above is not checked) I have reviewed the AI-generated content before submitting.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[x] Other, please explain:

This PR progresses to step 2 of the ecosystem tests rollout plan.

What changes did you make? (Give an overview)

• Added main to push.branches so the workflow runs on every push to main
• Removed continue-on-error: true so test failures are reported as real build failures
• Updated the Discord notification condition to use failure() instead of steps.tester.outcome

Refs #20819

Is there anything you'd like reviewers to focus on?