Avoid polluting global namespace in TypesExplainer tests

[Bo98]

Description

Setting constants in a spec does not limit it to that spec - it will pollute the global namespace and bleed over to other tests.

In Ruby 4.0, this issue becomes apparent as it breaks different tests with:

uninitialized constant YARD::Tags::TypesExplainer::Parser::Ruby

You could fix this elsewhere by fully qualifying any reference to Parser but the real underlying problem here is the namespace pollution.

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).