Update Expert insights mode use app cache

[Steve-Mcl]

Description

This pull request refactors the Expert MCP access token caching system to use the application's centralized cache abstraction instead of the direct lru-cache dependency. It also updates affected API routes and tests to handle the now-async cache methods. These changes improve consistency, reliability, and testability of token caching and invalidation.

Caching system refactor:

• Replaced direct usage of lru-cache with the application's cache abstraction (app.caches) in forge/expert/index.js, including new helper methods for cache access and token expiry handling. Now, token caching, retrieval, and clearing are all async and use the shared cache infrastructure. [1] [2] [3]

API updates:

• Updated forge/routes/api/expert.js and forge/routes/api/project.js to await the now-async token cache methods, ensuring proper cache handling in API endpoints. [1] [2]

Test updates:

• Updated all relevant tests in test/unit/forge/routes/api/expert_spec.js to await the async cache methods, ensuring tests correctly interact with the new caching logic. [1] [2] [3] [4] [5] [6] [7]

Related Issue(s)

closes #7137

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 76.57%. Comparing base (9dd545e) to head (7f83e85).
⚠️ Report is 2 commits behind head on 1178-app-cache-ttl.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           1178-app-cache-ttl    #7138   +/-   ##
===================================================
  Coverage               76.57%   76.57%           
===================================================
  Files                     405      405           
  Lines                   20382    20384    +2     
  Branches                 4905     4902    -3     
===================================================
+ Hits                    15607    15609    +2     
  Misses                   4775     4775           

Flag	Coverage Δ
backend	76.57% <100.00%> (+<0.01%)	⬆️

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