Summary

This PR implements the centralized MQTT Manager for Catroid, responsible for the full connection lifecycle of an MQTT broker client:

• CATROID-1671: MqttManager — connect, disconnect, state tracking, and failure handling

⚠️ Dependency notice: This PR depends on the MQTT settings PR covering CATROID-1670 and CATROID-1672 (mqtt-settings-entry-task-tdd). The dependency commits have been cherry-picked here to keep this branch buildable independently. This PR should only be merged after that PR is merged. Once it is, this branch will be rebased and the cherry-picked commits removed.

This ticket is scoped to connection management only. Message publishing, subscriptions, and MQTT bricks are deferred to separate tickets.

CATROID-1671 — MqttManager for Broker Connection Lifecycle

Changes Implemented

org.catrobat.catroid.devices.mqtt

• MqttClientInterface
  Abstracts the Paho client behind an interface, decoupling MqttManager from the third-party library and enabling pure JVM unit tests without an Android runtime or real network.

• PahoMqttClient
  Thin adapter wrapping org.eclipse.paho.client.mqttv3.MqttClient behind MqttClientInterface. Used in production.

• MqttConnectionConfig
  Data class grouping all six broker parameters (host, port, clientId, username, password, TLS flag). Includes a fromContext() factory that reads values from SettingsFragment. Keeps connect() to a single overload with one parameter.

• MqttManager
  Core manager class:

  • Singleton via by lazy — follows the same pattern as RaspberryPiService
  • connect(config) — establishes broker connection, idempotent if already connected, validates host, auto-generates client ID if blank
  • connectFromContext(context) — convenience entry point reading config from SettingsFragment
  • disconnect() — graceful disconnect and resource cleanup, idempotent if not connected
  • isConnected — queryable connection state backed by the client
  • buildServerUri() — produces tcp:// or ssl:// URI based on TLS flag
  • buildConnectOptions() — sets clean session, timeout, and optional credentials
  • On connect failure: logs error, calls close() on the client to release any allocated resources, nulls the reference — app never crashes

Testing

Added MqttManagerTest with 26 unit tests using FakeMqttClient (hand-written test double — no Mockito, no emulator, pure JVM):

• Singleton: instance is non-null and returns same object on repeated access
• Initial state: isConnected is false before any call
• connect() success path: returns true, calls client, sets callback
• connect() failure path: returns false without crashing when MqttException thrown, calls close() for resource cleanup
• connect() guard paths: already connected returns true without reconnecting; blank host returns false without touching client; empty client ID auto-generates one
• URI building: tcp:// vs ssl:// scheme selection, full URI format
• Connect options: isCleanSession always true, credentials set only when non-blank
• disconnect(): calls disconnect() and close() on client; skips both when not connected; safe to call twice

Followed TDD workflow:

• Red
• Green
• Refactor

26 unit tests added.

Static Analysis

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