- Add lib/errors.js with 21 documented error codes
- Update all modules to use centralized error constants
- Change inconsistent 'NoAuth' code to 'ENOAUTH'
- Add error codes to previously uncoded errors in:
  - sendmail-transport (ESENDMAIL)
  - xoauth2 (EOAUTH2)
  - fetch (EFETCH)
  - http-proxy-client (EPROXY)
  - mailer (EPROXY)
  - mime-node (EFILEACCESS, EURLACCESS)
  - nodemailer (ECONFIG)
- Add test/errors/errors-test.js

BREAKING CHANGE: Error code 'NoAuth' renamed to 'ENOAUTH'

- Clear socket timeout before destruction to prevent timer leaks
- Explicitly remove all socket event listeners in close() method
- Remove error listener from plain socket after TLS upgrade

When DNS returns multiple A/AAAA records for an SMTP host, the
connection now tries alternative addresses if the initial connection
fails. This improves reliability when some resolved addresses are
unreachable.

Changes:
- Resolve both IPv4 and IPv6 addresses and expose via _addresses
- Add _connectToHost and _onConnectionError methods for fallback logic
- Fix event listener cleanup by using stored handler references
- Add tests for connection fallback scenarios

- Add connection attempt tracking to prevent stale callbacks from executing
  during fallback retries
- Replace removeAllListeners() with targeted removeListener() to preserve
  TLS internal listeners during socket cleanup

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>