docs(agentchat): add missing TextMessage import in custom agents docs

[Jane Illarionova (JaneIllario)]

Summary

This PR fixes a documentation issue in the AgentChat Custom Agents guide.

Specifically, the ArithmeticAgent example uses TextMessage but did not import it in the same code cell.
This update adds the missing import so the snippet is self-consistent and runnable when copied.

Changes

• Updated python/docs/src/user-guide/agentchat-user-guide/custom-agents.ipynb
• In the ArithmeticAgent import section:
  • changed:
    • from autogen_agentchat.messages import BaseChatMessage
  • to:
    • from autogen_agentchat.messages import BaseChatMessage, TextMessage

Issue

Fixes #6277

Notes

• Documentation-only change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.23%. Comparing base (027ecf0) to head (5c3b186).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7676   +/-   ##
=======================================
  Coverage   81.23%   81.23%           
=======================================
  Files         244      244           
  Lines       18527    18527           
=======================================
  Hits        15050    15050           
  Misses       3477     3477           

Flag	Coverage Δ
unittests	81.23% <ø> (ø)

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