Use a root json element instead of dict

[jackgerrits]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 41.66667% with 7 lines in your changes missing coverage. Please review.

Project coverage is 76.48%. Comparing base (3c30d89) to head (4a123d5).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/Microsoft.AutoGen/Core.Grpc/GrpcAgentRuntime.cs	0.00%	3 Missing and 2 partials ⚠️
dotnet/src/Microsoft.AutoGen/Core/BaseAgent.cs	0.00%	1 Missing ⚠️
...net/src/Microsoft.AutoGen/Core/InProcessRuntime.cs	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5430      +/-   ##
==========================================
- Coverage   78.08%   76.48%   -1.60%     
==========================================
  Files         158      192      +34     
  Lines        9576    10606    +1030     
  Branches        0      138     +138     
==========================================
+ Hits         7477     8112     +635     
- Misses       2099     2400     +301     
- Partials        0       94      +94     

Flag	Coverage Δ
unittests	76.48% <41.66%> (-1.60%)	⬇️

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