Narrow return types for AbstractQuery::getSingleScalarResult()

[phansys]

Tests were also improved:

• QueryTest::testGetSingleScalarResultThrowsExceptionOnNonUniqueResult() was updated to ensure NonUniqueResultException is thrown only on results with multiple rows;
• QueryTest::testGetSingleScalarResultThrowsExceptionOnSingleRowWithMultipleColumns() was added to ensure NonUniqueResultException is thrown only on results with multiple columns; <- These changes were moved to Fix and improve functional test cases expecting NonUniqueResultException from AbstractQuery::getSingleScalarResult() #10722 (see Narrow return types for AbstractQuery::getSingleScalarResult() #10721 (review)).

Closes #10695

[greg0ire]

There should be 2 commits if you are doing unrelated things. Also, mixed is technically correct, so the commit about changing the phpdoc should target 2.16.x (while the other commit should target 2.15.x to reduce differences, since it does not touch lib).
I've noticed that scalar is a type that exist for static analysis: https://psalm.dev/docs/annotating_code/type_syntax/scalar_types/#scalar

I'm undecided on whether we should use it or not, I would be OK with any solution.

[phansys]

There should be 2 commits if you are doing unrelated things.

TIL, I never created a PR with commits targeting different branches.

I've noticed that scalar is a type that exist for static analysis

I didn't used scalar in order to keep support for other tools that only recognize native types.

[greg0ire]

Thanks @phansys !