JSON serialization of a stream

Program.cs

using System.Buffers.Text;
using System.Text.Json;
using System.Text.Json.Serialization;

var text = "Hello, World! How are ya? Good! That's swell!"u8;
{
	using var ms = new MemoryStream(text.ToArray());
	var foo = new Foo("text", ms);
	using var fs = File.Create("yes.txt", 4096, FileOptions.WriteThrough);
	JsonSerializerOptions jso = new() { Converters = { new StreamToBase64WriteOnlyJsonConverter(fs) }};
	using var writer = new Utf8JsonWriter(fs);
	JsonSerializer.Serialize(writer, foo, jso);
}
var json = File.ReadAllText("yes.txt");
Console.WriteLine(json);
Console.WriteLine(Convert.ToBase64String(text));

record Foo(string Name, Stream Reader);

public sealed class StreamToBase64WriteOnlyJsonConverter(Stream writeStream) : JsonConverter<Stream>
{
	public override Stream Read(ref Utf8JsonReader r, Type ttc, JsonSerializerOptions o) => throw new NotImplementedException();

	public override void Write(Utf8JsonWriter writer, Stream value, JsonSerializerOptions options)
	{
		writer.WriteNumberValue(0); // write a dummy value so the writer knows it has written a value
		writer.Flush(); // flush the writer to the underlying stream
		writeStream.Position -= 1; // move the stream back one byte to overwrite the dummy value
		
		writeStream.Write("\""u8);
		Span<byte> buffer = stackalloc byte[4];
		var maxNonBase64Bytes = Base64.GetMaxDecodedFromUtf8Length(buffer.Length);
		int read;
		while ((read = value.Read(buffer[..maxNonBase64Bytes])) != 0)
		{
			Base64.EncodeToUtf8InPlace(buffer, read, out var bytesWritten);
			writeStream.Write(buffer[..bytesWritten]);
		}
		writeStream.Write("\""u8);
	}
}