Overhaul tool handling (#89)

* Overhaul tool handling

- Renames [McpTool{Type}] to [McpServerTool{Type}], in order to distinguish representations of tools on the server from tools on the client.
- Enables [McpServerTool] methods to have arguments injected from DI, as well as a specific set of types from the implementation directly, like IMcpServer.
- Renames WithTools to WithToolsFromAssembly.
- All of the WithToolsXx methods now publish each individual McpServerTool into DI; other code can do so as well. The options setup code gathers all of the tools from DI and combines them into a collection which is then stored in McpServerOptions and used to construct the server.
- The server tools specified via DI as well as manually-provided handlers, using CallToolHandler as a fallback in case the requested tool doesn't exist in the tools collection, and ListToolsHandler augments the information for whatever tools exist.
- The tools are stored in McpServerOptions in a new McpServerToolCollection type, which is a thread-safe container that exposes add/removal notification. Adding/removing tools triggers a change notification that in turn sends a notification to the client about a tools update.
- The ServerOptions are exposed from the server instance.
- Removed cursor-based APIs from McpClientExtensions.
- Changed McpClientExtensions APIs to return `Task<IList<...>` rather than `IAsyncEnumerable<...>`.

I'm sure this will need to evolve further before we're "done", but it's a significant improvement from where we are now. One area I'm not super happy with is the notifying collection; that might work ok for a stdio server, where you can grab the created collection from the server's options and mutate it, but I don't know how well that's going to work for sse servers.

* Make McpServerTool{Collection} extensible

* Fix README.md example

* Add notifications test

* Address feedback

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -1,4 +1,5 @@
 using Microsoft.Extensions.AI;
+using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol.Messages;
@@ -24,7 +25,7 @@ public class McpServerTests : LoggedTest
         _serverTransport = new Mock<IServerTransport>();
         _logger = new Mock<ILogger>();
         _options = CreateOptions();
-        _serviceProvider = new Mock<IServiceProvider>().Object;
+        _serviceProvider = new ServiceCollection().BuildServiceProvider();
     }
 
     private static McpServerOptions CreateOptions(ServerCapabilities? capabilities = null)
@@ -133,8 +134,7 @@ public class McpServerTests : LoggedTest
         await transport.SendMessageAsync(new JsonRpcNotification
             {
                 Method = "notifications/initialized"
-            }
-, TestContext.Current.CancellationToken);
+            }, TestContext.Current.CancellationToken);
 
         await Task.Delay(50, TestContext.Current.CancellationToken);
 
@@ -678,12 +678,15 @@ public class McpServerTests : LoggedTest
         public bool IsInitialized => throw new NotImplementedException();
 
         public Implementation? ClientInfo => throw new NotImplementedException();
-        public IServiceProvider? ServiceProvider => throw new NotImplementedException();
+        public McpServerOptions ServerOptions => throw new NotImplementedException();
+        public IServiceProvider? Services => throw new NotImplementedException();
         public void AddNotificationHandler(string method, Func<JsonRpcNotification, Task> handler) => 
             throw new NotImplementedException();
         public Task SendMessageAsync(IJsonRpcMessage message, CancellationToken cancellationToken = default) =>
             throw new NotImplementedException();
         public Task StartAsync(CancellationToken cancellationToken = default) =>
             throw new NotImplementedException();
+
+        public object? GetService(Type serviceType, object? serviceKey = null) => null;
     }
 }