Test Slack workflow failure notification

[danielporterda]

What changed

Adds a single explicit failure step at the start of the Validate network variable tabs pull-request workflow.

Why

This is a disposable smoke test for .github/workflows/notify-slack-on-failure.yml. The default-branch notification workflow subscribes to completed failures from Validate network variable tabs and should post the failed run to the configured SLACK_WEBHOOK_URL.

Expected result

• Validate network variable tabs fails at Fail intentionally to test Slack workflow notifications.
• Notify Slack on Any Workflow Failure runs after that failure and succeeds if the webhook invocation is accepted.
• A Slack message appears with the failed workflow name, branch, actor, and run link.

Observed result

• The intentional validation failure occurred in run 31022408337.
• The follow-on notification completed successfully in run 31022423154, including the Send Slack notification step.
• Human confirmation that the message appeared in the destination Slack channel is still needed.

Cleanup

Do not merge this PR. Close it and delete the branch after the Slack message is confirmed (or after the notification failure is diagnosed).

Local checks

• Workflow YAML parses successfully.
• git diff --check passes.
• Commit includes a DCO sign-off.