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.

src/ModelContextProtocol/Client/McpClient.cs

@@ -49,7 +49,7 @@ internal sealed class McpClient : McpEndpoint, IMcpClient
         {
             if (capabilities.NotificationHandlers is { } notificationHandlers)
             {
-                NotificationHandlers.AddRange(notificationHandlers);
+                NotificationHandlers.RegisterRange(notificationHandlers);
             }
 
             if (capabilities.Sampling is { } samplingCapability)

src/ModelContextProtocol/Client/McpClientExtensions.cs

@@ -600,7 +600,7 @@ public static class McpClientExtensions
             var progressToken = requestParams.Meta?.ProgressToken;
 
             List<ChatResponseUpdate> updates = [];
-            await foreach (var update in chatClient.GetStreamingResponseAsync(messages, options, cancellationToken))
+            await foreach (var update in chatClient.GetStreamingResponseAsync(messages, options, cancellationToken).ConfigureAwait(false))
             {
                 updates.Add(update);