add missing unit tests

[wanglam]

Description

1.Add unit tests for notebook utils
2.Add unit tests for message content
3.Add unit tests for chat context

Issues Resolved

[List any issues this PR will resolve]

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 (d4290b0) 93.45% compared to head (7578827) 95.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   93.45%   95.61%   +2.15%     
==========================================
  Files          46       46              
  Lines        1253     1253              
  Branches      299      299              
==========================================
+ Hits         1171     1198      +27     
+ Misses         80       53      -27     
  Partials        2        2              

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