Reject invalid upload chunk sizes before opening a channel

[OskarEichler]

Summary

Reject zero, negative and non-Integer upload chunk sizes before opening an SSH channel. Preserve the existing nil/false default behavior.

Reproduction and evidence

Standalone local reproduction:

require 'net/scp'
require 'stringio'
session = Object.new
def session.logger; nil; end
def session.open_channel; raise 'channel was opened'; end
Net::SCP.new(session).upload(StringIO.new('abc'), 'out', chunk_size: 0)
# before: channel was opened; after: ArgumentError

Zero-sized reads repeatedly produce an empty String and never reach EOF. Focused/model checks reject 0, negative integers, floats, strings and arbitrary objects before channel creation, and accept positive integers and the existing defaults.

Compatibility / breaking changes

Intentional validation change: previously coerced non-Integer values (for example 1.5) now raise ArgumentError. Zero/negative invalid values fail earlier instead of hanging/failing after channel setup. Positive Integer behavior is 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.