Enhancement 7123: add rule for subscribers output

[CrochetFeve0251]

Description

Fixes #7123
Handle the return type from the get_subscribed_events method and makes sure the format is right.

Type of change

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

Detailed scenario

Run phpstan.

Technical description

Documentation

It creates a new PHPStan rule to process the code.

New dependencies

None

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.)

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00% (target: -0.10%)	✅ ∅ (target: 50.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (df52190)	38228	16731	43.77%
Head commit (962e6ff)	38228 (+0)	16731 (+0)	43.77% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#7138)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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