Restore SMTP IO sync even when DATA flushing fails

[OskarEichler]

Summary

Restore the original IO sync setting in an ensure around DATA flushing. A flush exception currently bypasses restoration and leaves the IO in buffered mode. Keep the existing flush operation and exception precedence.

Reproduction

require 'net/smtp'
io = Object.new
class << io
  attr_accessor :sync
  def flush = raise(IOError, 'synthetic flush failure')
end
io.sync = true
socket = Struct.new(:io).new(io)
def socket.write_message(*) = nil
smtp = Net::SMTP.new('example.invalid', starttls: false)
smtp.instance_variable_set(:@socket, socket)
smtp.define_singleton_method(:get_response) { |_| Net::SMTP::Response.parse('354 continue') }
begin
  smtp.data('local fixture')
rescue IOError
end
p io.sync # before: false; after: true

Verification

• 64 focused checks cover initial sync true/false, String/block message forms, write/flush failures separately and together, successful messages, exactly one flush, and original error object identity. Four failed expectations before; zero after.
• Existing rake test: 95 tests, 170 assertions, zero failures/errors, on baseline and this isolated branch, Ruby 4.0.6 via rbenv. Existing local TLS cases are included.
• Supplemental RuboCop Lint has the same six existing findings; Ruby syntax and git diff --check pass. No new/modified repository tests, dependencies or workflow settings.
• Independent branch based on c2e9102e1f199a5a10e5261a776d752e64cc130f; runtime matches released 0.5.1. External focused checks were used under the consumer repository's no-new-tests policy.

Compatibility and limits

No signature or protocol change. The original sync mode is now restored after flush failure. A flush error still takes precedence over an earlier write error; this patch does not make a failed SMTP exchange resumable. A sync= implementation that itself raises remains outside this fix. Other Ruby/OS runtimes were not run locally. No production or external SMTP delivery was used. Maintainer CI status is separate from these local checks.