Handle Pathname in download/upload

[til]

This implements the suggested solution 2.) from #102 for handling Pathname instances in net-sftp.

[OskarEichler]

I verified the current PR head and found a direct-load regression: the transfer operation files now reference Pathname, but neither file requires it, so loading either operation directly can raise NameError. A minimal follow-up is available at OskarEichler@9cd2519 and can be cherry-picked. With that fixup, the focused attribute checks (3 runs / 24 assertions), v5 base checks (194 / 374), and full suite (435 / 1,199) pass apart from the pre-existing Ruby 4 StartTest keyword expectation failure.