feat: save/load test for dotnet agents

[bassmang]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 23.80952% with 16 lines in your changes missing coverage. Please review.

Project coverage is 76.32%. Comparing base (25f26a3) to head (e27f272).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/Microsoft.AutoGen/Core.Grpc/GrpcAgentRuntime.cs	0.00%	9 Missing and 3 partials ⚠️
...net/src/Microsoft.AutoGen/Core/InProcessRuntime.cs	62.50%	1 Missing and 2 partials ⚠️
dotnet/src/Microsoft.AutoGen/Core/BaseAgent.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5284      +/-   ##
==========================================
+ Coverage   76.24%   76.32%   +0.07%     
==========================================
  Files         192      192              
  Lines       10601    10609       +8     
  Branches      138      140       +2     
==========================================
+ Hits         8083     8097      +14     
+ Misses       2421     2416       -5     
+ Partials       97       96       -1     

Flag	Coverage Δ
unittests	76.32% <23.80%> (+0.07%)	⬆️

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