refactor: fix Session and SessionInterface code

[paulbalandan]

Description
While looking at #9534 to find where the culprit is (in markAsTempData()), I found some points for improvements for the Session class:

• Improve array PHPDocs by indicating their keys and values
• array|bool|float|int|object|string|null is just the long form of mixed. If the type cannot be narrowed further (like string|int|null), I see no benefit in writing the long form instead of just mixed.
• Simplify separate array and single operations

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

I noticed some potential problems in the markAsFlashdata() method.

[paulbalandan]

Rebased to show the added tests pass on this refactor.