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/SseIntegrationTests.cs

@@ -213,9 +213,9 @@ public class SseIntegrationTests(ITestOutputHelper outputHelper) : LoggedTest(ou
             {
                 Capabilities = new()
                 {
-                    NotificationHandlers = [new("test/notification", args =>
+                    NotificationHandlers = [new("test/notification", (notification, cancellationToken) =>
                     {
-                        var msg = args.Params?["message"]?.GetValue<string>();
+                        var msg = notification.Params?["message"]?.GetValue<string>();
                         receivedNotification.SetResult(msg);
 
                         return Task.CompletedTask;