[JsonSchemaExporter] TreatNullObliviousAsNonNullable incorrectly applies to Nullable<T> types

[eiriktsarpalis]

Description

Discovered while investigating modelcontextprotocol/csharp-sdk#601

Reproduction Steps

using System.Text.Json;
using System.Text.Json.Schema;

JsonSchemaExporterOptions options = new() { TreatNullObliviousAsNonNullable = true };
Console.WriteLine(JsonSerializerOptions.Default.GetJsonSchemaAsNode(typeof(int?), exporterOptions: options));

Expected behavior

Should have produced

{
  "type": ["integer","null"]
}

Actual behavior

Actually produces

{
  "type": "integer"
}

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.