Extensions are not being deserialized into server capabilities

[darena-mhaque]

Describe the bug
When setting extensions in server capabilities, they are not being included in the initialize message.

To Reproduce
Steps to reproduce the behavior:

1. Start up a very minimal asp.net server with the following (add an extension):

using System.Text.Json.Nodes;
using ModelContextProtocol.Protocol;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
// Learn more about configuring OpenAPI at https://aka.ms/aspnet/openapi
builder.Services.AddOpenApi();
builder
    .Services.AddMcpServer(options =>
    {
        options.Capabilities ??= new ServerCapabilities();

#pragma warning disable MCPEXP001
        options.Capabilities.Extensions ??= new Dictionary<string, object>();
        options.Capabilities.Extensions.Add("io.myext", new JsonObject { ["required"] = true });
#pragma warning restore MCPEXP001
    })
    .WithHttpTransport();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.MapOpenApi();
}

app.UseHttpsRedirection();
app.MapMcp("/mcp");
app.Run();

2. Use postman, send a json request. Observe extensions are not included:

Expected behavior
The server capabilities should have the extensions property, which should include io.myext as configured.

Logs

Additional context