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

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

src/ModelContextProtocol.Core/Server/ISseEventStreamStore.cs

@@ -0,0 +1,23 @@
+namespace ModelContextProtocol.Server;
+
+/// <summary>
+/// Provides storage and retrieval of SSE event streams, enabling resumability and redelivery of events.
+/// </summary>
+public interface ISseEventStreamStore
+{
+    /// <summary>
+    /// Creates a new SSE event stream with the specified options.
+    /// </summary>
+    /// <param name="options">The configuration options for the new stream.</param>
+    /// <param name="cancellationToken">A token to cancel the operation.</param>
+    /// <returns>A writer for the newly created event stream.</returns>
+    ValueTask<ISseEventStreamWriter> CreateStreamAsync(SseEventStreamOptions options, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Gets a reader for an existing event stream based on the last event ID.
+    /// </summary>
+    /// <param name="lastEventId">The ID of the last event received by the client, used to resume from that point.</param>
+    /// <param name="cancellationToken">A token to cancel the operation.</param>
+    /// <returns>A reader for the event stream, or <c>null</c> if no matching stream is found.</returns>
+    ValueTask<ISseEventStreamReader?> GetStreamReaderAsync(string lastEventId, CancellationToken cancellationToken = default);
+}