fix: 修复 App 端 MQTT 运行时兼容问题

- 锁定 MQTT 版本为 5.15.1
- 为 AbortController、AbortSignal 和 navigator.language 添加兼容补丁
- 新增 MQTT App 运行时测试

scripts/test-mqtt-app-runtime.js

@@ -0,0 +1,49 @@
+const fs = require('fs')
+const path = require('path')
+const vm = require('vm')
+
+const bundle = fs.readFileSync(
+  path.join(path.dirname(require.resolve('mqtt/package.json')), 'dist/mqtt.js'),
+  'utf8'
+)
+
+const context = vm.createContext({
+  console,
+  setTimeout,
+  clearTimeout,
+  setInterval,
+  clearInterval,
+  queueMicrotask,
+  TextEncoder,
+  TextDecoder,
+  URL,
+})
+
+vm.runInContext(bundle, context, { timeout: 10000 })
+
+if (typeof context.AbortController !== 'function') {
+  throw new Error('MQTT bundle did not install AbortController fallback')
+}
+if (typeof context.AbortSignal !== 'function') {
+  throw new Error('MQTT bundle did not install AbortSignal fallback')
+}
+
+const controller = new context.AbortController()
+let abortEvents = 0
+controller.signal.addEventListener('abort', () => {
+  abortEvents += 1
+}, { once: true })
+controller.abort('test-reason')
+controller.abort('ignored-reason')
+
+if (!controller.signal.aborted) {
+  throw new Error('AbortSignal was not marked aborted')
+}
+if (controller.signal.reason !== 'test-reason') {
+  throw new Error('AbortSignal reason was not preserved')
+}
+if (abortEvents !== 1) {
+  throw new Error(`Expected one abort event, got ${abortEvents}`)
+}
+
+console.log('MQTT App runtime fallback test passed')