Expert/mqtt FrontEnd scaffolding

[cstns]

Description

This PR scaffolds frontend MQTT support and restructures service startup/cleanup into a centralized orchestrator, while updating dependent code and adding tests.

• Replaces the old service.factory with a new services.orchestrator that boots and tears down frontend services more cleanly.
• Adds a new, substantial mqtt.service.js that manages MQTT connections, publishing, subscribing, lifecycle cleanup, payload serialization, and validation.
• Renames the messaging service class/file to post-message.service.js and updates consumers accordingly.
• Updates app bootstrap (main.js) to use the new orchestrator and initialize services through it.
• Adjusts a drawer helper and product assistant store to reference the new service setup.
• Adds @types/mqtt to dev dependencies.
• Includes extensive unit tests for the new MQTT service and related service wiring.

Related Issue(s)

closes #6962

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.29%. Comparing base (07d9e5e) to head (eb660d2).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7004   +/-   ##
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  Coverage   76.29%   76.29%           
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  Files         403      403           
  Lines       20296    20296           
  Branches     4881     4881           
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  Hits        15485    15485           
  Misses       4811     4811           

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