Streamable HTTP resumability + redelivery + SSE polling via server-side disconnect (#1077)

Co-authored-by: Stephen Halter <halter73@gmail.com>

src/ModelContextProtocol.AspNetCore/SseEventStreamReaderExtensions.cs

@@ -0,0 +1,53 @@
+using ModelContextProtocol.Protocol;
+using ModelContextProtocol.Server;
+using System.Buffers;
+using System.Net.ServerSentEvents;
+using System.Text.Json;
+using System.Text.Json.Serialization.Metadata;
+
+namespace ModelContextProtocol.AspNetCore;
+
+/// <summary>
+/// Provides extension methods for <see cref="ISseEventStreamReader"/>.
+/// </summary>
+internal static class SseEventStreamReaderExtensions
+{
+    /// <summary>
+    /// Copies all events from the reader to the destination stream in SSE format.
+    /// </summary>
+    /// <param name="reader">The event stream reader to copy events from.</param>
+    /// <param name="destination">The destination stream to write SSE-formatted events to.</param>
+    /// <param name="cancellationToken">A token to cancel the operation.</param>
+    /// <returns>A task that represents the asynchronous copy operation.</returns>
+    /// <exception cref="ArgumentNullException">Thrown when <paramref name="reader"/> or <paramref name="destination"/> is null.</exception>
+    public static async Task CopyToAsync(this ISseEventStreamReader reader, Stream destination, CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(reader);
+        ArgumentNullException.ThrowIfNull(destination);
+
+        Utf8JsonWriter? jsonWriter = null;
+        var jsonTypeInfo = (JsonTypeInfo<JsonRpcMessage>)McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonRpcMessage));
+
+        var events = reader.ReadEventsAsync(cancellationToken);
+        await SseFormatter.WriteAsync(events, destination, FormatEvent, cancellationToken).ConfigureAwait(false);
+
+        void FormatEvent(SseItem<JsonRpcMessage?> item, IBufferWriter<byte> writer)
+        {
+            if (item.Data is null)
+            {
+                return;
+            }
+
+            if (jsonWriter is null)
+            {
+                jsonWriter = new Utf8JsonWriter(writer);
+            }
+            else
+            {
+                jsonWriter.Reset(writer);
+            }
+
+            JsonSerializer.Serialize(jsonWriter, item.Data, jsonTypeInfo);
+        }
+    }
+}