Search root agent id by rootAgentName specified in opensearch_dashboards.yml

[gaobinlong]

Description

This PR aims to change the behavior of getting root agent id, users have to specify the name of the root agent in the yml, and when they send the first message in the chat box, the node server will call the search agent api in ml-commons to get the root agent id, if multiple root agents are found, we only fetch the latest one.

This PR also lets users to change the root agent dynamically, they can delete the old root agent firstly and create a new one with the old name, when sending message, the api in backend will throw 404 error with message resource_not_found_exception, this time the node server will retry to search the root agent by the specified name, if found, the new root agent id will be used and cached.

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

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (510b685) 93.26% compared to head (60abf5a) 93.44%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #86      +/-   ##
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+ Coverage   93.26%   93.44%   +0.17%     
==========================================
  Files          46       46              
  Lines        1218     1251      +33     
  Branches      291      298       +7     
==========================================
+ Hits         1136     1169      +33     
  Misses         80       80              
  Partials        2        2              

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