chore: Add missing is_response to LangChain completion messages

[jsumners-nr]

This PR fixes an oversight from previous work.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.16%. Comparing base (4d0d839) to head (9cf69c2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2044   +/-   ##
=======================================
  Coverage   97.16%   97.16%           
=======================================
  Files         248      248           
  Lines       41648    41655    +7     
=======================================
+ Hits        40466    40473    +7     
  Misses       1182     1182           

Flag	Coverage Δ
integration-tests-16.x	78.50% <44.44%> (+<0.01%)	⬆️
integration-tests-18.x	78.46% <44.44%> (-0.02%)	⬇️
integration-tests-20.x	78.48% <44.44%> (+<0.01%)	⬆️
unit-tests-16.x	90.80% <66.66%> (-0.01%)	⬇️
unit-tests-18.x	90.79% <66.66%> (-0.01%)	⬇️
unit-tests-20.x	90.79% <66.66%> (-0.01%)	⬇️
versioned-tests-16.x	74.91% <44.44%> (-0.03%)	⬇️
versioned-tests-18.x	75.50% <100.00%> (-0.02%)	⬇️
versioned-tests-20.x	75.51% <100.00%> (-0.02%)	⬇️

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