Update chat service due to refactor

[ananzh]

Description

Adapt to the refactor in chat opensearch-project/OpenSearch-Dashboards#10983
Test video is added in the above PR

Issues Resolved

NA

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.26%. Comparing base (ed6c382) to head (ddef8d2).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #247   +/-   ##
=======================================
  Coverage   66.26%   66.26%           
=======================================
  Files          58       58           
  Lines        2007     2007           
  Branches      481      485    +4     
=======================================
  Hits         1330     1330           
  Misses        670      670           
  Partials        7        7           

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