Reject nested SMTP starts before entering session cleanup

[OskarEichler]

Summary

Reject an already-started SMTP session before entering block-form start cleanup. Previously the IOError was raised by do_start inside the ensure scope, so an unsuccessful nested start sent QUIT and closed the caller's active session.

Reproduction

require 'net/smtp'
smtp = Net::SMTP.new('example.invalid', starttls: false)
# Isolate cleanup without an external SMTP server.
socket = Object.new
def socket.closed? = true
def socket.close = nil
smtp.instance_variable_set(:@socket, socket)
smtp.instance_variable_set(:@started, true)
begin
  smtp.start { raise 'must not yield' }
rescue IOError => e
  p e.message
end
p smtp.started? # before: false; after: true
p smtp.instance_variable_get(:@socket).equal?(socket) # false -> true

Verification

• 18 focused checks include bounded real localhost sessions: both block and non-block repeated starts raise the same IOError, never yield, retain the original connection, and allow an actual RSET afterward. Normal block return/cleanup is retained. Baseline: five failed expectations; patched: zero.
• 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 signature or exception-class/message change. An unsuccessful nested start no longer closes the already-active session. Argument-count validation retains precedence. Related open #86 proposes a broader quit/disconnect API; this patch adds no such API. 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.