Add locked field test

[hardillb]

Description

Test for #4198

Related Issue(s)

#4198

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.14%. Comparing base (ffe5ece) to head (9fa2f5a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4280   +/-   ##
=======================================
  Coverage   78.13%   78.14%           
=======================================
  Files         292      292           
  Lines       13403    13403           
  Branches     3000     3000           
=======================================
+ Hits        10473    10474    +1     
+ Misses       2930     2929    -1     

Flag	Coverage Δ
backend	78.14% <ø> (+<0.01%)	⬆️

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