Fix Extensions not being copied to ServerCapabilities during initialization

[Copilot]

• Fix ConfigureExperimental in McpServerImpl.cs to copy Extensions from options to ServerCapabilities
• Use public Extensions property instead of internal ExtensionsCore
• Remove redundant #pragma warning disable MCPEXP001 in test (already suppressed in tests/Directory.Build.props)
• Rename ConfigureExperimental → ConfigureExperimentalAndExtensions
• Remove redundant = new() from ServerCapabilities property (already set in ctor)
• Verify no equivalent issue on client side
• Add test verifying Extensions in initialize response
• Add test verifying Experimental in initialize response
• Add reflection-based test to catch future missing capability copies
• Build and tests passing

Original prompt

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This section details on the original issue you should resolve

<issue_title>Extensions are not being deserialized into server capabilities</issue_title>
<issue_description>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

</issue_description>

<agent_instructions>It looks like in

csharp-sdk/src/ModelContextProtocol.Core/Server/McpServerImpl.cs

Lines 85 to 93 in 1578e94

	ServerCapabilities = new();
	ConfigureInitialize(options);
	ConfigureTools(options);
	ConfigurePrompts(options);
	ConfigureResources(options);
	ConfigureTasks(options);
	ConfigureLogging(options);
	ConfigureCompletion(options);
	ConfigureExperimental(options);

we're failing to copy Experimental over to the server's used options from the input options. Please fix, check if there's an equivalent issue on the client side (and fix it if there is), and ensure this scenario is tested. Also, see if there's a good way to add a test that will help us catch such issues in the future when we add additional capabilities and forget to copy them; maybe something using reflection (but only public reflection if that's the best route, not private reflection).</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Extensions are not being deserialized into server capabilities #1398

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