feat: improve assert_broadcast_on error message

[StephaneRob]

Motivation / Background

This Pull Request has been created to improve error message when using assert_broadcast_on. Indeed it easier to debug if the error message contains messages that have been broadcasted (or information that there is no message at all)
See #47202

Detail

This Pull Request changes the assert_broadcast_on error message to display messages that have been broadcasted to a given channel

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.