Initialise message_handler in bv_refinement unit test

[tautschnig]

The message_handler member of bv_refinementt::infot was left uninitialised, which is undefined behaviour when it is later dereferenced. Set it to null_message_handler.

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[Copilot]

Pull request overview

This PR fixes undefined behavior in a JBMC unit test by ensuring bv_refinementt::infot::message_handler is initialized before being dereferenced.

Changes:

• Initialize bv_refinementt::infot::message_handler to null_message_handler in the instantiate_not_contains unit test helper.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.41%. Comparing base (eee45a7) to head (ec7cebe).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8863      +/-   ##
===========================================
+ Coverage    80.00%   80.41%   +0.40%     
===========================================
  Files         1703     1703              
  Lines       188397   188398       +1     
  Branches        73       73              
===========================================
+ Hits        150736   151496     +760     
+ Misses       37661    36902     -759     

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