Fix Extensions not being copied to ServerCapabilities and add tests

In McpServerImpl.ConfigureExperimental, the Extensions property from
the options was not being copied to the server's ServerCapabilities.
This caused Extensions to be missing from the initialize response.

Also added:
- Test for Extensions in initialize response
- Test for Experimental in initialize response
- Reflection-based test to catch future missing capability copies

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -265,6 +265,7 @@ private void ConfigureCompletion(McpServerOptions options)
     private void ConfigureExperimental(McpServerOptions options)
     {
         ServerCapabilities.Experimental = options.Capabilities?.Experimental;
+        ServerCapabilities.ExtensionsCore = options.Capabilities?.ExtensionsCore;
     }
 
     private void ConfigureResources(McpServerOptions options)

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -290,6 +290,95 @@ await Can_Handle_Requests(
             });
     }
 
+#pragma warning disable MCPEXP001
+    [Fact]
+    public async Task Initialize_IncludesExtensionsInResponse()
+    {
+        await Can_Handle_Requests(
+            serverCapabilities: new ServerCapabilities
+            {
+                Extensions = new Dictionary<string, object> { ["io.myext"] = new JsonObject { ["required"] = true } },
+            },
+            method: RequestMethods.Initialize,
+            configureOptions: null,
+            assertResult: (_, response) =>
+            {
+                var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
+                Assert.NotNull(result);
+                Assert.NotNull(result.Capabilities.Extensions);
+                Assert.True(result.Capabilities.Extensions.ContainsKey("io.myext"));
+            });
+    }
+
+    [Fact]
+    public async Task Initialize_IncludesExperimentalInResponse()
+    {
+        await Can_Handle_Requests(
+            serverCapabilities: new ServerCapabilities
+            {
+                Experimental = new Dictionary<string, object> { ["customFeature"] = new JsonObject { ["enabled"] = true } },
+            },
+            method: RequestMethods.Initialize,
+            configureOptions: null,
+            assertResult: (_, response) =>
+            {
+                var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
+                Assert.NotNull(result);
+                Assert.NotNull(result.Capabilities.Experimental);
+                Assert.True(result.Capabilities.Experimental.ContainsKey("customFeature"));
+            });
+    }
+
+    [Fact]
+    public async Task Initialize_CopiesAllCapabilityProperties()
+    {
+        // Set every public property on ServerCapabilities to a non-null value.
+        // If a new property is added to ServerCapabilities in the future but the
+        // server fails to copy it, this test will fail as a reminder.
+        var inputCapabilities = new ServerCapabilities
+        {
+            Experimental = new Dictionary<string, object> { ["test"] = new JsonObject() },
+            Logging = new LoggingCapability(),
+            Prompts = new PromptsCapability(),
+            Resources = new ResourcesCapability(),
+            Tools = new ToolsCapability(),
+            Completions = new CompletionsCapability(),
+            Tasks = new McpTasksCapability(),
+            Extensions = new Dictionary<string, object> { ["io.test"] = new JsonObject() },
+        };
+
+        await Can_Handle_Requests(
+            serverCapabilities: inputCapabilities,
+            method: RequestMethods.Initialize,
+            configureOptions: options =>
+            {
+                // Tasks capability requires a TaskStore
+                options.TaskStore = new InMemoryMcpTaskStore();
+            },
+            assertResult: (_, response) =>
+            {
+                var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
+                Assert.NotNull(result);
+
+                // Use reflection to verify every public property on ServerCapabilities is non-null.
+                // This catches cases where new capability properties are added but not copied
+                // from options in McpServerImpl.
+                foreach (var property in typeof(ServerCapabilities).GetProperties(BindingFlags.Public | BindingFlags.Instance))
+                {
+                    if (!property.CanRead)
+                    {
+                        continue;
+                    }
+
+                    Assert.True(
+                        property.GetValue(result.Capabilities) is not null,
+                        $"ServerCapabilities.{property.Name} was set on options but is null in the initialize response. " +
+                        $"Ensure the property is copied in McpServerImpl's Configure* methods.");
+                }
+            });
+    }
+#pragma warning restore MCPEXP001
+
     [Fact]
     public async Task Can_Handle_Completion_Requests()
     {