feat(storage): add comprehensive tests for StorageManager with memory, filesystem, and custom function backends

feat(email): implement EmailSendJob class for robust email delivery with retry logic and MX record resolution

feat(mail): restructure mail module exports for simplified access to core and delivery functionalities

ts/mail/delivery/classes.unified.rate.limiter.ts

@@ -1,4 +1,5 @@
 import * as plugins from '../../plugins.ts';
+import { EventEmitter } from 'node:events';
 import { logger } from '../../logger.ts';
 import { SecurityLogger, SecurityLogLevel, SecurityEventType } from '../../security/index.ts';
 
@@ -83,7 +84,7 @@ export interface IRateLimitResult {
 /**
  * Unified rate limiter for all email processing modes
  */
-export class UnifiedRateLimiter extends plugins.EventEmitter {
+export class UnifiedRateLimiter extends EventEmitter {
   private config: IHierarchicalRateLimits;
   private counters: Map<string, ILimitCounter> = new Map();
   private patternCounters: Map<string, ILimitCounter> = new Map();