Fix message ID for correlation between streaming chunks and final mes…

[smalltalkman]

…sage.

Why are these changes needed?

Related issue number

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.99%. Comparing base (8e2c5f0) to head (444067c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6969   +/-   ##
=======================================
  Coverage   80.99%   80.99%           
=======================================
  Files         237      237           
  Lines       18196    18198    +2     
=======================================
+ Hits        14737    14739    +2     
  Misses       3459     3459           

Flag	Coverage Δ
unittests	80.99% <100.00%> (+<0.01%)	⬆️

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