.NET: Add support for background responses to A2A agent #7310

Job	Run time
paths-filter	10s
dotnet-build-and-test (net9.0, windows-latest, Debug)	11m 27s
dotnet-build-and-test (net10.0, ubuntu-latest, Release, true, integration)	7m 47s
dotnet-build-and-test (net8.0, ubuntu-latest, Release)	6m 44s
dotnet-build-and-test (net472, windows-latest, Release, true, integration)	10m 41s
dotnet-build-and-test-check	3s
	36m 52s

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