refactor: reduce PHPStan child return type baseline

[memleakd]

Description
Refines PHPDoc return types so they are covariant with parent/interface contracts and better match the current implementations.

This reduces method.childReturnType PHPStan baseline entries from 28 to 2. The remaining entries are UploadedFile::move() and OutgoingRequest::getUri(), which are real API contract conflicts and should be handled separately.

Related to #6310.

Verification:

• composer phpstan:check
• git diff --check

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide