Retry device api call when tunnel proxy returns 502 to re-set the device affinity token

[cstns]

Description

The Load Balancer (LB) occasionally delegates requests to stale or incorrect tunnel instances. Because the LB uses sticky sessions, once a "bad" path is established, subsequent requests fail until the affinity is reset.

Addresses the ~50% failure rate where the Load Balancer routes requests to improper tunnel backends. This PR introduces a retry mechanism that refreshes the device affinity token upon encountering an HTTP 502.

Statistical Impact (5 Attempts)
Initial Call 50%
Retry 1 75%
Retry 2 87.5%
Retry 3 93.75%
Retry 4 96.875%

Related Issue(s)

closes #6747

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 76.65%. Comparing base (6a2bf39) to head (0f744ff).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6748   +/-   ##
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  Coverage   76.65%   76.65%           
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  Files         398      398           
  Lines       20108    20108           
  Branches     4844     4844           
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  Hits        15414    15414           
  Misses       4694     4694           

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