Allow notification handlers post-creation temporary registration (#223)

- We still allow notification handlers to be configured pre-creation, eliminating any race conditions that may result from missed notifications. But now we also allow for post-creation notification handler registration.
- Handlers may now also be removed. This permits temporary registrations that may be created in a scoped manner.
- Registration handling is thread-safe.
- Handlers are now cancelable.
- Exceptions from notification handlers trigger all normal exception handling in the dispatch pipeline.

This also:
- Adds a WithStreamServerTransport as a counterpart to WithStdioServerTransport. The latter was used in several tests in a hacky way, and that's now simplified via the former's existence.

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -637,6 +637,8 @@ public class McpServerTests : LoggedTest
             throw new NotImplementedException();
         public Task RunAsync(CancellationToken cancellationToken = default) =>
             throw new NotImplementedException();
+        public IAsyncDisposable RegisterNotificationHandler(string method, Func<JsonRpcNotification, CancellationToken, Task> handler) =>
+            throw new NotImplementedException();
     }
 
     [Fact]
@@ -648,7 +650,7 @@ public class McpServerTests : LoggedTest
         var notificationReceived = new TaskCompletionSource<JsonRpcNotification>();
         options.Capabilities = new()
         {
-            NotificationHandlers = [new(NotificationMethods.ProgressNotification, notification =>
+            NotificationHandlers = [new(NotificationMethods.ProgressNotification, (notification, cancellationToken) =>
             {
                 notificationReceived.TrySetResult(notification);
                 return Task.CompletedTask;