feat: Implement Deno-native STARTTLS handler and connection wrapper

- Refactored STARTTLS implementation to use Deno's native TLS via Deno.startTls().
- Introduced ConnectionWrapper to provide a Node.js net.Socket-compatible interface for Deno.Conn and Deno.TlsConn.
- Updated TlsHandler to utilize the new STARTTLS implementation.
- Added comprehensive SMTP authentication tests for PLAIN and LOGIN mechanisms.
- Implemented rate limiting tests for SMTP server connections and commands.
- Enhanced error handling and logging throughout the STARTTLS and connection upgrade processes.

test/suite/smtpserver_commands/test.cmd-04.data-command.test.ts

@@ -157,14 +157,11 @@ Deno.test({
       await sendSmtpCommand(conn, 'MAIL FROM:<sender@example.com>', '250');
 
       // Try DATA after MAIL FROM but before RCPT TO
-      // NOTE: Current server implementation accepts DATA without RCPT TO (returns 354)
-      // RFC 5321 suggests this should be rejected with 503, but some servers allow it
+      // RFC 5321: DATA must only be accepted after RCPT TO
       const response = await sendSmtpCommand(conn, 'DATA');
-      assertMatch(response, /^(354|503)/, 'Server responds to DATA (354=accept, 503=reject)');
+      assertMatch(response, /^503/, 'Should reject DATA before RCPT TO with 503');
 
-      if (response.startsWith('354')) {
-        console.log('⚠️  Server accepts DATA without RCPT TO (non-standard but allowed)');
-      }
+      console.log('✓ DATA before RCPT TO correctly rejected with 503');
     } finally {
       try {
         await closeSmtpConnection(conn);