Incorrect serialization Code is Generated For Extensible Enums of Union Type

[jorgerangel-msft]

When an union model property type is customized to an extensible enum of more than 1 type, ie public readonly partial struct HyperparameterCycleCount : IEquatable<int>, IEquatable<string> the underlying model is generating deserialization code that doesn't compile.

.

Similarly, the serialization code is generated incorrectly.

Sample spec: https://github.com/mikhail/openai-in-typespec/blob/c22818b1e6d61ffde44d19501e0ebacb13559eb9/.typespec/fine-tuning/custom.tsp#L5.
Sample generated code: https://github.com/mikhail/openai-in-typespec/blob/c22818b1e6d61ffde44d19501e0ebacb13559eb9/.dotnet/src/Generated/Models/HyperparameterOptions.Serialization.cs#L97

[jorgerangel-msft]

Possible solution: Check the custom enum type to see what union value types are supported to generate a deserialization block like so:

HyperparameterCycleCount cycleCount = default;
if (prop.NameEquals("n_epochs"u8))
{
    if (prop.Value.ValueKind == JsonValueKind.Null)
    {
        cycleCount = null;
    }
    else if (prop.Value.ValueKind == JsonValueKind.String)
    {
        cycleCount = new HyperparameterCycleCount(prop.Value.GetString());
    }
    else
    {
        cycleCount = new HyperparameterCycleCount(prop.Value.GetInt32());
    }
 
    continue;
}

cc: @JoshLove-msft