tests(source-generator): additional tests for complex scenarios

[Meir017]

from https://github.com/dotnet/orleans/actions/runs/14795101967/job/41540349265?pr=9470

Module	Line	Branch	Method
Orleans.CodeGenerator	72.84%	66.36%	73.44%

orleans-source-generator-coverage.zip

not quite sure what source-code would trigger this -

orleans/src/Orleans.CodeGenerator/SerializerGenerator.cs

Lines 150 to 153 in 63480a5

	case FieldAccessorDescription accessor when accessor.InitializationSyntax == null:
	//[UnsafeAccessor(UnsafeAccessorKind.Method, Name = "set_Amount")]
	//extern static void SetAmount(External instance, int value);
	return

and this

orleans/src/Orleans.CodeGenerator/CopierGenerator.cs

Lines 130 to 133 in 63480a5

	case FieldAccessorDescription accessor when accessor.InitializationSyntax == null:
	//[UnsafeAccessor(UnsafeAccessorKind.Method, Name = "set_Amount")]
	//extern static void SetAmount(External instance, int value);
	return

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