Add default expert URLs

[hardillb]

fixes #6895

Description

Adds default URLs for assistant and expert using the new unified hostname expert.flowfuse.com.

NOT TO BE MERGED BEFORE: https://github.com/FlowFuse/CloudProject/pull/1224

Related Issue(s)

#6895

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 (872d5df) to head (17adff5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6949   +/-   ##
=======================================
  Coverage   76.29%   76.29%           
=======================================
  Files         403      403           
  Lines       20293    20293           
  Branches     4880     4883    +3     
=======================================
  Hits        15482    15482           
  Misses       4811     4811           

Flag	Coverage Δ
backend	76.29% <100.00%> (ø)

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[hardillb]

@Steve-Mcl I've had to remove 2 tests as there won't be a case of the URLs not being set anymore.

Hopefully this is just temporary until https://github.com/FlowFuse/engineering/issues/6 is fixed, when suitable new tests can be added.