Check if we can rely on PHP to save the session for us

Problem/Motivation

Follow-up from 📌 Avoid reading session from the database multiple times during a request Needs work
@longwave wrote:

Symfony's SessionInterface::save() says

* This method is generally not required for real sessions as
* the session will be automatically saved at the end of
* code execution.

so do we have to explicitly save the session ourselves at all?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet