feat: add feature flag for chat v2 endpoint in LearnerAppConfig

[mraman-2U]

This pull request introduces a small configuration change to the learner application. The main update is the addition of a new feature flag to control the use of a chat v2 endpoint.

• Added FEATURE_ENABLE_CHAT_V2_ENDPOINT to the app configuration, allowing the chat v2 endpoint feature to be toggled via environment variables.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.83%. Comparing base (af50d5a) to head (d4e85b0).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1792   +/-   ##
=======================================
  Coverage   90.83%   90.83%           
=======================================
  Files         344      344           
  Lines        5792     5794    +2     
  Branches     1331     1333    +2     
=======================================
+ Hits         5261     5263    +2     
  Misses        514      514           
  Partials       17       17           

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