Reset SMTP error state before opening a fresh session

[OskarEichler]

Summary

Clear the previous critical-section error flag before opening a fresh SMTP session. A failed non-block start otherwise leaves the flag set; a later start can report success while skipping the new connection's greeting, EHLO and subsequent commands.

Reproduction

require 'net/smtp'
# A deterministic offline reproduction of the same failed-greeting state.
smtp = Net::SMTP.new('example.invalid', starttls: false)
socket = Object.new
def socket.close = nil
smtp.define_singleton_method(:tcp_socket) { |*| socket }
smtp.define_singleton_method(:new_internet_message_io) { |s| s }
greetings = 0
smtp.define_singleton_method(:recv_response) do
  greetings += 1
  raise EOFError if greetings == 1
  Net::SMTP::Response.parse('220 fixture')
end
smtp.define_singleton_method(:do_helo) { |*| nil }
begin
  smtp.start
rescue EOFError
end
smtp.start
p greetings # before: 1; after: 2
# No real network or delivery occurs in this isolated reproduction.

Verification

• 16 focused checks also use a bounded real localhost listener: the first connection closes before its greeting; retrying the same SMTP object (block and non-block forms) reads the second greeting, transmits EHLO, records SIZE and sends RSET. Baseline falsely reports RSET success without transmitting it. Six 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 public API change and no automatic retry is introduced. Only a caller-initiated new session clears the previous error state. Failed exchanges on an existing session remain non-resumable; this patch does not reset the documented ESMTP fallback setting. 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.