Enhancement 7123: add rule for subscribers output

[CrochetFeve0251]

Description

Fixes #7123 Explain how this code impacts users.

Type of change

• [ ] Enhancement (non-breaking change which improves an existing functionality).

Detailed scenario

Explain how to reproduce the issue (for bugs) or how to trigger the new code. You can refer to the GitHub issue or grooming if it is still applicable.

Technical description

Documentation

Explain how this code works. Diagrams & drawings are welcome.

New dependencies

List any new dependencies that are required for this change.

Risks

List possible performance & security issues or risks, and explain how they have been mitigated.

Mandatory Checklist

Code validation

• [ ] I validated all the Acceptance Criteria. If possible, provide screenshots or videos.
• [ ] I triggered all changed lines of code at least once without new errors/warnings/notices.
• [ ] I implemented built-in tests to cover the new/changed code.

Code style

• [ ] I wrote a self-explanatory code about what it does.
• [ ] I protected entry points against unexpected inputs.
• [ ] I did not introduce unnecessary complexity.
• [ ] Output messages (errors, notices, logs) are explicit enough for users to understand the issue and are actionnable.

Additional Checks

• [ ] In the case of complex code, I wrote comments to explain it.
• [ ] When possible, I prepared ways to observe the implemented system (logs, data, etc.).
• [ ] I added error handling logic when using functions that could throw errors (HTTP/API request, filesystem, etc.)