Fix incorrect recurrence display for events with invalid schedules

[MiguelAxcar]

Overview

This pull request is address issue reported in #52, where a cron event with invalid schedule was showing invalid information in the recurrence column.

Changes

• A Test-Driven Development (TDD) approach is used for write tests first, reproducing the issue.
• Code has been updated for handling scenario where schedule was previously valid but no longer exists.
• If recurrence schedule for event is missing, recurrence column now display "Non-repeating", adhering to user expectations.

Details

• Modified Cron_Event_Command class to check if schedule exists before formatting recurrence.
• Added scenario in feature tests for cover both cases where schedule exists and where it not.

Testing

The pull request include new test cases that cover described scenario, ensure that recurrence display is handled correct.

Impact

This fix make better reliability of wp cron event list command, providing accurate informations about event recurrence.

I kindly request review of these changes, and I'm available for any further discussion or adjustments as needed.

I would like to also thank @schlessera and @danielbachhuber for the support provided during 2023 WordCamp US Contributor Day.

Related wp-cli/wp-cli#5832

[danielbachhuber]

Great work on this, @MiguelAxcar ! Way to stick with it 💪