fix: avoid node:domain emit crash on connection close

[SAY-5]

fixes #351

SMTPConnection extends EventEmitter but keeps a string domain field. Once anything loads the deprecated node:domain module (e.g. node:repl on node 24, or an explicit import on node 26), EventEmitter.usingDomains flips on and emit() starts calling this.domain.enter(). Because our domain is a string, that throws TypeError: domain.enter is not a function, which surfaces on connection.close() after sendAsync.

This renames the internal field to _domain so it no longer shadows the property the domain module manages. The public domain constructor option is unchanged. Added a regression test that loads node:domain and emits an event on a connection.