JsonValueReaderWriter.FromJsonString doesn't seem to correctly parse

[roji]

var readerWriter2 = JsonInt32ReaderWriter.Instance;

var i = readerWriter2.ToJsonString(8); // i is "8"
var j = (int)readerWriter2.FromJsonString(i);

Stack trace:

Cannot get the value of a token type 'None' as a number.
   at System.Text.Json.ThrowHelper.ThrowInvalidOperationException_ExpectedNumber(JsonTokenType tokenType)
   at System.Text.Json.Utf8JsonReader.TryGetInt32(Int32& value)
   at System.Text.Json.Utf8JsonReader.GetInt32()
   at Microsoft.EntityFrameworkCore.Storage.Json.JsonInt32ReaderWriter.FromJsonTyped(Utf8JsonReaderManager& manager, Object existingObject) in /home/roji/projects/efcore/src/EFCore/Storage/Json/JsonInt32ReaderWriter.cs:line 24

The problem seems to be that ToJsonString() doesn't read the first tokenbefore calling into FromJson().

FromJsonString() is used in CollectionToJsonStringConverter (and only there), that may be broken as a result.

/cc @ajcvickers