Decouple Handlers and Collections from protocol types (#765)

* Decouple Handlers and Collections (e.g. ToolsHandler and ToolsCollection) from Capabilities by promoting them to the options types.

Also, group all handlers in container classes, for server, I decided to move McpServerHandlers down to Mcp.Core and move the NotificationHandlers there too.

Also, Fix ListResourceTemplatesHandler issue.

* Add remarks for empty protocol capability types

* Keep JsonIgnore properties and mark them as Obsolete

* Continue supporting Obsolete members

* Update Obsolete messages to indicate the member will be removed

* Add EditorBrowsable Never to the remaining obsolete members

src/ModelContextProtocol.Core/Client/McpClientHandlers.cs

@@ -0,0 +1,88 @@
+using Microsoft.Extensions.AI;
+using ModelContextProtocol.Protocol;
+
+namespace ModelContextProtocol.Client;
+
+/// <summary>
+/// Provides a container for handlers used in the creation of an MCP client.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This class provides a centralized collection of delegates that implement various capabilities of the Model Context Protocol.
+/// </para>
+/// <para>
+/// Each handler in this class corresponds to a specific client endpoint in the Model Context Protocol and
+/// is responsible for processing a particular type of message. The handlers are used to customize
+/// the behavior of the MCP server by providing implementations for the various protocol operations.
+/// </para>
+/// <para>
+/// When a server sends a message to the client, the appropriate handler is invoked to process it
+/// according to the protocol specification. Which handler is selected
+/// is done based on an ordinal, case-sensitive string comparison.
+/// </para>
+/// </remarks>
+public class McpClientHandlers
+{
+    /// <summary>Gets or sets notification handlers to register with the client.</summary>
+    /// <remarks>
+    /// <para>
+    /// When constructed, the client will enumerate these handlers once, which may contain multiple handlers per notification method key.
+    /// The client will not re-enumerate the sequence after initialization.
+    /// </para>
+    /// <para>
+    /// Notification handlers allow the client to respond to server-sent notifications for specific methods.
+    /// Each key in the collection is a notification method name, and each value is a callback that will be invoked
+    /// when a notification with that method is received.
+    /// </para>
+    /// <para>
+    /// Handlers provided via <see cref="NotificationHandlers"/> will be registered with the client for the lifetime of the client.
+    /// For transient handlers, <see cref="IMcpEndpoint.RegisterNotificationHandler"/> may be used to register a handler that can
+    /// then be unregistered by disposing of the <see cref="IAsyncDisposable"/> returned from the method.
+    /// </para>
+    /// </remarks>
+    public IEnumerable<KeyValuePair<string, Func<JsonRpcNotification, CancellationToken, ValueTask>>>? NotificationHandlers { get; set; }
+
+    /// <summary>
+    /// Gets or sets the handler for <see cref="RequestMethods.RootsList"/> requests.
+    /// </summary>
+    /// <remarks>
+    /// This handler is invoked when a client sends a <see cref="RequestMethods.RootsList"/> request to retrieve available roots.
+    /// The handler receives request parameters and should return a <see cref="ListRootsResult"/> containing the collection of available roots.
+    /// </remarks>
+    public Func<ListRootsRequestParams?, CancellationToken, ValueTask<ListRootsResult>>? RootsHandler { get; set; }
+
+    /// <summary>
+    /// Gets or sets the handler for processing <see cref="RequestMethods.ElicitationCreate"/> requests.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This handler function is called when an MCP server requests the client to provide additional
+    /// information during interactions. The client must set this property for the elicitation capability to work.
+    /// </para>
+    /// <para>
+    /// The handler receives message parameters and a cancellation token.
+    /// It should return a <see cref="ElicitResult"/> containing the response to the elicitation request.
+    /// </para>
+    /// </remarks>
+    public Func<ElicitRequestParams?, CancellationToken, ValueTask<ElicitResult>>? ElicitationHandler { get; set; }
+
+    /// <summary>
+    /// Gets or sets the handler for processing <see cref="RequestMethods.SamplingCreateMessage"/> requests.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This handler function is called when an MCP server requests the client to generate content
+    /// using an AI model. The client must set this property for the sampling capability to work.
+    /// </para>
+    /// <para>
+    /// The handler receives message parameters, a progress reporter for updates, and a 
+    /// cancellation token. It should return a <see cref="CreateMessageResult"/> containing the 
+    /// generated content.
+    /// </para>
+    /// <para>
+    /// You can create a handler using the <see cref="McpClientExtensions.CreateSamplingHandler"/> extension
+    /// method with any implementation of <see cref="IChatClient"/>.
+    /// </para>
+    /// </remarks>
+    public Func<CreateMessageRequestParams?, IProgress<ProgressNotificationValue>, CancellationToken, ValueTask<CreateMessageResult>>? SamplingHandler { get; set; }
+}