added TextMessage dependency in custom-agents.ipynb

[swapnilxi]

Why are these changes needed?

Related issue number

Closes #6277

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[victordibia]

@swapnilxi , it seems your changes may have corrupted the notebook.
Can you confirm the notebook is valid at your end?

[swapnilxi]

adding that line worked fine for me for the missing depencency

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.27%. Comparing base (eca80ff) to head (fd37b42).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6276   +/-   ##
=======================================
  Coverage   77.27%   77.27%           
=======================================
  Files         200      200           
  Lines       14322    14322           
=======================================
  Hits        11068    11068           
  Misses       3254     3254           

Flag	Coverage Δ
unittests	77.27% <ø> (ø)

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