Deny ACL check for deleted MQTT Clients

[hardillb]

Fixes Sentry error

Description

Deleted clients trying to publish/subscribe to new topics.

Related Issue(s)

https://flowfuse.sentry.io/issues/6061657489/?referrer=slack&notification_uuid=ca9e75cc-86c3-46d0-bae7-e82be18e94dd&alert_rule_id=14700011&alert_type=issue

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

Labels

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

[knolleary]

A failing test to investigate.

[hardillb]

Fix had code from USN feature branch, removed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.79%. Comparing base (b42992f) to head (a7982b6).
Report is 46 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4766   +/-   ##
=======================================
  Coverage   78.79%   78.79%           
=======================================
  Files         311      311           
  Lines       14781    14782    +1     
  Branches     3384     3385    +1     
=======================================
+ Hits        11646    11647    +1     
  Misses       3135     3135           

Flag	Coverage Δ
backend	78.79% <100.00%> (+<0.01%)	⬆️

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