[csharp][generichost] Fix DateOnly parsing #236

Job	Run time
Build .Net projects (samples/client/petstore/csharp/generichost/net9/AnyOf)	38s
Build .Net projects (samples/client/petstore/csharp/generichost/latest/OneOfList)	37s
Build .Net projects (samples/client/petstore/csharp/generichost/net9/AllOf)	35s
Build .Net projects (samples/client/petstore/csharp/generichost/latest/Tags)	37s
Build .Net projects (samples/client/petstore/csharp/generichost/net9/AnyOfNoCompare)	36s
Build .Net projects (samples/client/petstore/csharp/generichost/latest/HelloWorld)	39s
Build .Net projects (samples/client/petstore/csharp/generichost/net9/FormModels)	40s
Build .Net projects (samples/client/petstore/csharp/generichost/net9/Petstore)	40s
Build .Net projects (samples/client/petstore/csharp/generichost/net9/NullReferenceTypes)	42s
Build .Net projects (samples/client/petstore/csharp/generichost/latest/ComposedEnum)	33s
Build .Net projects (samples/client/petstore/csharp/generichost/latest/InlineEnumAnyOf)	34s
Build .Net projects (samples/client/petstore/csharp/generichost/net9/OneOf)	39s
Build .Net projects (samples/client/petstore/csharp/httpclient/net9/Petstore)	38s
Build .Net projects (samples/client/petstore/csharp/httpclient/net9/Petstore-nonPublicApi)	45s
Build .Net projects (samples/client/petstore/csharp/generichost/net9/SourceGeneration)	47s
Build .Net projects (samples/client/petstore/csharp/restsharp/net9/EnumMappings)	37s
Build .Net projects (samples/client/petstore/csharp/generichost/net9/UseDateTimeForDate)	36s
	10m 53s

Provide feedback