Fix Missing ChatCompletion events in LangChain when using MCP

[lrafeei]

This PR resolves the issue of missing ChatCompletion events when using LangChain/LangGraph with MCP tools, specifically when the response format is not the default JSON.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	9		0	0	1.07s
✅ MARKDOWN	markdownlint	7	0	0	0	1.44s
✅ PYTHON	ruff	1088	0	0	0	1.05s
✅ PYTHON	ruff-format	1088	0	0	0	0.43s
✅ YAML	prettier	23	0	0	0	1.74s
✅ YAML	v8r	23		0	0	6.28s
✅ YAML	yamllint	23		0	0	0.81s

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.07%. Comparing base (2778c17) to head (3a83b9c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1800      +/-   ##
==========================================
- Coverage   82.07%   82.07%   -0.01%     
==========================================
  Files         217      217              
  Lines       26864    26867       +3     
  Branches     4255     4254       -1     
==========================================
+ Hits        22048    22050       +2     
- Misses       3341     3343       +2     
+ Partials     1475     1474       -1     

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