Draft: PoC: Fix /-/readiness probe in Puma Single

What does this MR do?

PoC: fix /readiness probe for Puma in a Single mode.

Not intended to be merged, I opened it to understand possible callbacks/hooks refactoring better.

Even though it works, it clearly signalizes that our LifecycleEvents needs some rework.

The main reasons are:

• in Puma Single, on_master_start & on_worker_start are getting ambiguous. Most of the time they intend to mean the same in a Single mode (the Puma process starts), but in this particular case we don't want to execute on_worker_start in a non-clustered environment. This makes this code extremely confusing and error-prone.
• on_before_fork doesn't make sense in Puma Single

I'll open a separate issue for callbacks/hooks rework: #320749 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Related to #300661 (closed)