refactor: LangGraph LlmAgent event, remove old LlmEvent base

[amychisholm03]

Description

Refactors the LangGraph LlmAgent event to use the new LlmEvent base (event-base.js) and adds missing assertions to LangGraph tests.

Removes old event base (event.js) and sets intended private function in LlmErrorMessage to actually be private.

How to Test

npm run unit
npm run versioned langgraph

Related Issues

Closes #3687

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.76%. Comparing base (8503d25) to head (be5c3bc).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3761      +/-   ##
==========================================
- Coverage   97.89%   97.76%   -0.13%     
==========================================
  Files         454      452       -2     
  Lines       58749    58490     -259     
  Branches        1        1              
==========================================
- Hits        57512    57183     -329     
- Misses       1237     1307      +70     

Flag	Coverage Δ
integration-tests-cjs-20.x	74.39% <72.22%> (+0.08%)	⬆️
integration-tests-cjs-22.x	74.42% <72.22%> (+0.08%)	⬆️
integration-tests-cjs-24.x	75.19% <72.22%> (+0.04%)	⬆️
integration-tests-esm-20.x	53.01% <72.22%> (-0.01%)	⬇️
integration-tests-esm-22.x	53.06% <72.22%> (-0.01%)	⬇️
integration-tests-esm-24.x	54.19% <72.22%> (-0.05%)	⬇️
unit-tests-20.x	89.44% <88.88%> (-0.05%)	⬇️
unit-tests-22.x	89.46% <88.88%> (-0.05%)	⬇️
unit-tests-24.x	89.46% <88.88%> (-0.05%)	⬇️
versioned-tests-20.x	81.33% <100.00%> (-0.36%)	⬇️
versioned-tests-22.x	81.33% <100.00%> (-0.36%)	⬇️
versioned-tests-24.x	81.24% <100.00%> (-0.37%)	⬇️

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