feat: expose underlying MQTT client through getMqttClient() method

- Add getMqttClient() method to both McpMqttServer and McpMqttClient
- Add getClient() method to UniversalMqttAdapter and MqttAdapter interface
- Update README with documentation for accessing underlying MQTT client
- Allows users to perform custom pub/sub operations using the same MQTT connection

Author: ysfscream

README.md

@@ -439,6 +439,18 @@ host: 'mqtts://broker.emqx.io:8883'
 
 ## Advanced Usage
 
+### Accessing Underlying MQTT Client
+
+Both server and client provide `getMqttClient()` method to access the underlying MQTT client for custom pub/sub operations:
+
+```typescript
+const mqttClient = server.getMqttClient() // or client.getMqttClient()
+if (mqttClient) {
+  mqttClient.subscribe('custom/topic', { qos: 1 })
+  mqttClient.publish('custom/topic', 'Hello World')
+}
+```
+
 ### Custom Tool Validation
 
 ```typescript

src/client/mcp-client.ts

@@ -1,4 +1,5 @@
 import { EventEmitter } from 'events'
+import type { MqttClient } from 'mqtt'
 import type {
   McpMqttClientConfig,
   MqttConnectionOptions,
@@ -500,6 +501,10 @@ export class McpMqttClient extends EventEmitter {
   isServerConnected(serverId: string): boolean {
     return this.connectedServers.has(serverId)
   }
+
+  getMqttClient(): MqttClient | null {
+    return this.mqttAdapter.getClient()
+  }
 }
 
 export function createMcpClient(config: McpMqttClientConfig): McpMqttClient {

src/server/mcp-server.ts

@@ -512,6 +512,10 @@ export class McpMqttServer extends EventEmitter {
   getConnectedClients(): string[] {
     return Array.from(this.connectedClients)
   }
+
+  getMqttClient() {
+    return this.mqttAdapter.getClient()
+  }
 }
 
 export function createMcpServer(config: McpMqttServerConfig): McpMqttServer {

src/shared/mqtt-adapter.ts

@@ -30,6 +30,7 @@ export interface MqttAdapter {
   on(event: 'connect' | 'disconnect' | 'error', callback: (...args: any[]) => void): void
   isConnected(): boolean
   getConnackProperties(): Record<string, any> | undefined
+  getClient(): MqttClient | null
 }
 
 export class UniversalMqttAdapter implements MqttAdapter {
@@ -209,4 +210,8 @@ export class UniversalMqttAdapter implements MqttAdapter {
   getConnackProperties(): Record<string, any> | undefined {
     return this.connackProperties
   }
+
+  getClient(): MqttClient | null {
+    return this.client
+  }
 }