Adapt OpenURI progress and content length callbacks to SCP

[OskarEichler]

Summary

Adapt SCP's four-argument progress callback to OpenURI's one-argument byte count, and invoke content_length_proc once even for empty files.

Reproduction and evidence

The adapter previously forwarded a strict OpenURI ->(bytes) { ... } directly to SCP. SCP calls it with channel, filename, received and total, raising ArgumentError: wrong number of arguments (given 4, expected 1). The focused reproduction now passes. Combined models verify empty, one-byte, binary and 20,000-byte transfers, first/final progress and exactly one content-length callback. The extra empty-file progress event does not duplicate content length.

Compatibility / breaking changes

Intentional callback correction: callers that relied on the undocumented four-argument SCP signature through OpenURI must instead use OpenURI's byte-count signature or the direct SCP API. content_length_proc, previously ignored, is now honored. Direct Net::SCP progress callbacks are unchanged.

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.