Fix: MQTT topic not updated when LoRa region changes

[Copilot]

When MQTT is active and the LoRa region is changed, the running MQTT instance continues publishing and subscribing under the old region's topic prefix (e.g. msh/US/… after switching away from US), because moduleConfig.mqtt.root was updated but the topic strings cached in the MQTT object were not.

Changes

• src/mqtt/MQTT.h / src/mqtt/MQTT.cpp — Introduce MQTT::reinitTopics():

  • Resets cryptTopic, jsonTopic, mapTopic to base values and prepends the current moduleConfig.mqtt.root
  • Updates isConfiguredForDefaultRootTopic
  • Disconnects from the broker so the next reconnect re-subscribes under the new prefix
  • Constructor now calls reinitTopics() instead of duplicating the same inline block

• src/modules/AdminModule.cpp — Call mqtt->reinitTopics() immediately after moduleConfig.mqtt.root is rewritten on a region change (guarded by !MESHTASTIC_EXCLUDE_MQTT)

• src/graphics/draw/MenuHandler.cpp — Call mqtt->reinitTopics() immediately after moduleConfig.mqtt.root is rewritten on a region change via the on-device menu (guarded by !MESHTASTIC_EXCLUDE_MQTT); also adds the required mqtt/MQTT.h include under the same guard

• test/test_mqtt/MQTT.cpp — Add test_reinitTopicsUpdatesOnRegionChange: starts MQTT with root msh/US, simulates a region change to msh/EU_868 via reinitTopics(), then asserts both the refreshed subscriptions and the corrected publish topic

🤝 Attestations

• I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • Heltec (Lora32) V3
  • LilyGo T-Deck
  • LilyGo T-Beam
  • RAK WisBlock 4631
  • Seeed Studio T-1000E tracker card
  • Other (please specify below)

[jp-bennett]

@copilot this patch misses the case where the region is changed via menuhandler.cpp

[Copilot]

@copilot this patch misses the case where the region is changed via menuhandler.cpp

Fixed in the latest commit — MenuHandler.cpp now calls mqtt->reinitTopics() (guarded by !MESHTASTIC_EXCLUDE_MQTT) immediately after moduleConfig.mqtt.root is rewritten on a region change, mirroring the same pattern already in AdminModule.cpp.

[Copilot]

Pull request overview

This PR fixes stale MQTT publish/subscribe topics after a LoRa region change by centralizing topic-prefix (root) handling inside the MQTT singleton and reapplying it whenever moduleConfig.mqtt.root is rewritten (e.g., msh/US → msh/EU_868). This aligns runtime MQTT behavior with the device’s updated region-derived root topic without requiring a reboot.

Changes:

• Added MQTT::reinitTopics() to rebuild cryptTopic/jsonTopic/mapTopic from moduleConfig.mqtt.root and force a broker disconnect so the next reconnect re-subscribes under the new prefix.
• Updated region-change flows (Admin config + on-device menu) to call mqtt->reinitTopics() immediately after rewriting moduleConfig.mqtt.root.
• Added a unit test validating that reinitTopics() refreshes subscriptions and the publish topic after a simulated region change.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/mqtt/MQTT.h	Declares MQTT::reinitTopics() as the public entry point for rebuilding topic strings after root changes.
src/mqtt/MQTT.cpp	Implements reinitTopics() and uses it in the constructor to avoid duplicated topic-init logic; disconnects to refresh subscriptions on reconnect.
src/modules/AdminModule.cpp	Calls mqtt->reinitTopics() after rewriting the default region-derived MQTT root during region changes.
src/graphics/draw/MenuHandler.cpp	Adds guarded MQTT include and calls mqtt->reinitTopics() after rewriting the region-derived MQTT root via the device UI.
test/test_mqtt/MQTT.cpp	Adds test_reinitTopicsUpdatesOnRegionChange to confirm refreshed subscriptions + corrected publish topic under the new root.