#62226 [13.x] Fix Paginator::links() return type

Paginator::links() returns $this->render(), which returns a View (an Htmlable), but its docblock says string. LengthAwarePaginator::links() and CursorPaginator::links() already document Htmlable. This one was missed when render() was updated in 22f29791b5.

This lets static analysis and IDEs type $paginator->links() correctly. Docblock only, no runtime change. Added a type assertion in types/Pagination/Paginator.php.

Description drafted with Claude Code.