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

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

src/ModelContextProtocol.Core/Server/ISseEventStreamReader.cs

@@ -0,0 +1,36 @@
+using ModelContextProtocol.Protocol;
+using System.Net.ServerSentEvents;
+
+namespace ModelContextProtocol.Server;
+
+/// <summary>
+/// Provides read access to an SSE event stream, allowing events to be consumed asynchronously.
+/// </summary>
+public interface ISseEventStreamReader
+{
+    /// <summary>
+    /// Gets the session ID associated with the stream being read.
+    /// </summary>
+    string SessionId { get; }
+
+    /// <summary>
+    /// Gets the ID of the stream.
+    /// </summary>
+    /// <remarks>
+    /// This value is guaranteed to be unique on a per-session basis.
+    /// </remarks>
+    string StreamId { get; }
+
+    /// <summary>
+    /// Gets the messages from the stream as an <see cref="IAsyncEnumerable{T}"/>.
+    /// </summary>
+    /// <param name="cancellationToken">A token to cancel the operation.</param>
+    /// <returns>An <see cref="IAsyncEnumerable{T}"/> of <see cref="SseItem{T}"/> containing JSON-RPC messages.</returns>
+    /// <remarks>
+    /// If the stream's mode is set to <see cref="SseEventStreamMode.Polling"/>, the returned
+    /// messages will only include the currently-available events starting at the last event ID specified
+    /// when the reader was created. Otherwise, the returned messages will continue until the associated
+    /// <see cref="ISseEventStreamWriter"/> is disposed.
+    /// </remarks>
+    IAsyncEnumerable<SseItem<JsonRpcMessage?>> ReadEventsAsync(CancellationToken cancellationToken = default);
+}