Validate kb and km for apiMaxLength

[Steve-Mcl]

Description

Ensures user input is nkb or nmb for apiMaxLength

Related Issue(s)

#3577

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

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.89%. Comparing base (35ddb79) to head (3042e07).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3603   +/-   ##
=======================================
  Coverage   41.89%   41.89%           
=======================================
  Files         625      625           
  Lines       24288    24288           
  Branches     5929     5929           
=======================================
  Hits        10176    10176           
  Misses      14112    14112           

Flag	Coverage Δ
backend	78.53% <100.00%> (ø)
frontend	1.92% <0.00%> (ø)

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