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/helpers/utils.ts

@@ -359,3 +359,36 @@ export async function retryOperation<T>(
 
   throw lastError!;
 }
+
+/**
+ * Upgrade SMTP connection to TLS using STARTTLS command
+ * @param conn - Active SMTP connection
+ * @param hostname - Server hostname for TLS verification
+ * @returns Upgraded TLS connection
+ */
+export async function upgradeToTls(conn: Deno.Conn, hostname: string = 'localhost'): Promise<Deno.TlsConn> {
+  const encoder = new TextEncoder();
+
+  // Send STARTTLS command
+  await conn.write(encoder.encode('STARTTLS\r\n'));
+
+  // Read response
+  const response = await readSmtpResponse(conn);
+
+  // Check for 220 Ready to start TLS
+  if (!response.startsWith('220')) {
+    throw new Error(`STARTTLS failed: ${response}`);
+  }
+
+  // Read test certificate for self-signed cert validation
+  const certPath = new URL('../../test/fixtures/test-cert.pem', import.meta.url).pathname;
+  const certPem = await Deno.readTextFile(certPath);
+
+  // Upgrade connection to TLS with certificate options
+  const tlsConn = await Deno.startTls(conn, {
+    hostname,
+    caCerts: [certPem], // Accept self-signed test certificate
+  });
+
+  return tlsConn;
+}