[7441] Consolidate broker credential generation

[n-lark]

Description

Extracts the duplicated find-and-destroy → generate → insert → return logic from the six createClientFor* functions into one shared createClient helper, leaving each as a thin wrapper frfr.

Related Issue(s)

Resolves #7441

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

❌ Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.56%. Comparing base (abe7b49) to head (0da171d).

Files with missing lines	Patch %	Lines
forge/db/controllers/BrokerClient.js	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7642      +/-   ##
==========================================
- Coverage   76.58%   76.56%   -0.02%     
==========================================
  Files         413      413              
  Lines       21845    21823      -22     
  Branches     5757     5752       -5     
==========================================
- Hits        16730    16709      -21     
+ Misses       5115     5114       -1     

Flag	Coverage Δ
backend	76.56% <96.66%> (-0.02%)	⬇️

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