fix: gRPC Agent Runtime Serialization Registration

[lokitoth]

We were registering the serializers when we already had a concrete type on the way into Publish or Send on the .NET side. However, in a xLang scenario, messages could originate from e.g. Python before being sent/published from .NET, resulting in no serializer being found.

This change adds a second-change registration and lookup when the agent is instantiated based on the IHandle implementations.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.50%. Comparing base (492b106) to head (51ed705).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/Microsoft.AutoGen/Core.Grpc/GrpcAgentRuntime.cs	62.50%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5513      +/-   ##
==========================================
- Coverage   78.35%   76.50%   -1.85%     
==========================================
  Files         167      206      +39     
  Lines        9923    11037    +1114     
  Branches        0      150     +150     
==========================================
+ Hits         7775     8444     +669     
- Misses       2148     2496     +348     
- Partials        0       97      +97     

Flag	Coverage Δ
unittests	76.50% <83.33%> (-1.85%)	⬇️

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