Initialize SMTP exception messages through the superclass

[OskarEichler]

Summary

Initialize the superclass exception message and use standard Exception rendering/copy behavior. SMTP errors currently override message without initializing Exception, so to_s loses the message and exception("replacement") continues returning the old message.

Reproduction

require 'net/smtp'
error = Net::SMTPFatalError.new('local failure')
p [error.message, error.to_s, error.exception('replacement').message]
# Before: ["local failure", "Net::SMTPFatalError", "local failure"]
# After:  ["local failure", "local failure", "replacement"]

Verification

• 336 focused checks across all six SMTP exception classes cover String/Response inputs, explicit and empty messages, response identity, replacement copying, original-message retention, backtraces and Marshal round trips. Sixty 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

The required response argument and message: keyword remain unchanged; this does not reopen the zero-argument constructor policy in closed #48. Related merged #42 added String construction. Intentional corrections: to_s now shows the message and exception(replacement) honors replacement. The default response message is captured at construction rather than re-read after later mutation of the response. 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.