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/Protocol/Transport/SseClientSessionTransport.cs

@@ -173,7 +173,7 @@ internal sealed class SseClientSessionTransport : TransportBase
     {
         try
         {
-            await CloseAsync();
+            await CloseAsync().ConfigureAwait(false);
         }
         catch (Exception)
         {

src/ModelContextProtocol/Protocol/Transport/SseResponseStreamTransport.cs

@@ -77,7 +77,7 @@ public sealed class SseResponseStreamTransport(Stream sseResponseStream, string
         }
 
         // Emit redundant "event: message" lines for better compatibility with other SDKs.
-        await _outgoingSseChannel.Writer.WriteAsync(new SseItem<IJsonRpcMessage?>(message, SseParser.EventTypeDefault), cancellationToken);
+        await _outgoingSseChannel.Writer.WriteAsync(new SseItem<IJsonRpcMessage?>(message, SseParser.EventTypeDefault), cancellationToken).ConfigureAwait(false);
     }
 
     /// <summary>
@@ -94,7 +94,7 @@ public sealed class SseResponseStreamTransport(Stream sseResponseStream, string
             throw new InvalidOperationException($"Transport is not connected. Make sure to call {nameof(RunAsync)} first.");
         }
 
-        await _incomingChannel.Writer.WriteAsync(message, cancellationToken);
+        await _incomingChannel.Writer.WriteAsync(message, cancellationToken).ConfigureAwait(false);
     }
 
     private static Channel<T> CreateBoundedChannel<T>(int capacity = 1) =>

src/ModelContextProtocol/Protocol/Types/Capabilities.cs

@@ -34,7 +34,7 @@ public class ClientCapabilities
     /// The client will not re-enumerate the sequence.
     /// </remarks>
     [JsonIgnore]
-    public IEnumerable<KeyValuePair<string, Func<JsonRpcNotification, Task>>>? NotificationHandlers { get; set; }
+    public IEnumerable<KeyValuePair<string, Func<JsonRpcNotification, CancellationToken, Task>>>? NotificationHandlers { get; set; }
 }
 
 /// <summary>

src/ModelContextProtocol/Protocol/Types/ServerCapabilities.cs

@@ -45,5 +45,5 @@ public class ServerCapabilities
     /// The server will not re-enumerate the sequence.
     /// </remarks>
     [JsonIgnore]
-    public IEnumerable<KeyValuePair<string, Func<JsonRpcNotification, Task>>>? NotificationHandlers { get; set; }
+    public IEnumerable<KeyValuePair<string, Func<JsonRpcNotification, CancellationToken, Task>>>? NotificationHandlers { get; set; }
 }