Close owned transfer files when the channel closes with an error

[OskarEichler]

Summary

Close library-owned upload/download files in an ensure clause when a channel closes, including when exit-status validation raises. Leave caller-owned IO open.

Reproduction and evidence

Closes #20. A download receiving C0644 6 file opens its destination; a subsequent nonzero channel exit raises Net::SCP::Error but previously left that file descriptor open. The public-entrypoint channel simulation now observes the original error and a closed file. Additional models cover upload/download, nonzero/missing exit status, already closed files and caller-owned File/StringIO behavior.

Compatibility / breaking changes

No API or dependency change; existing transfer errors remain errors. This handles channel-close cleanup, not all exceptions from user progress callbacks or an abandoned channel that is never closed. No broader lifecycle guarantee is claimed.

Verification and limits

• This focused branch passes the existing upstream suite: 36 tests / 70 assertions, Ruby 4.0.6, net-ssh 7.3.3, test-unit 3.7.8 and Mocha 2.1.0; all Ruby commands used rbenv.
• The combined release-based consumer candidate passes the suite with frozen string literals enabled, 10 focused cases, 2,906 model assertions, 24 URI-component assertions and 19 local scp/URI assertions. Combined evidence includes other separately proposed fixes and existing upstream frozen-string corrections; it is not attributed to this patch alone.
• No repository tests added/modified under the consuming project's explicit no-new-tests policy. Reproductions/models ran externally. No deployed servers, SSH connections or production data were used.
• All seven runtime files parse with the Ruby 2.6 parser; actual older-Ruby/Windows/remote-server matrices were not run locally. No upstream lint config exists; targeted lint has the same eight pre-existing warnings, with no new warnings.
• Existing upstream PRs/issues and contribution guidance were checked. Maintainer CI results are pending; no CI pass is claimed.