feat: implement closeStandaloneSSEStream

Implement the ability to close standalone GET SSE streams, enabling
polling behavior for server-initiated notifications (SEP-1699).

Changes:
- closeStandaloneSSEStream() now closes the GET stream by ending
  the response and removing from stream mapping
- Events are now stored even when standalone stream is disconnected,
  allowing replay on client reconnect with Last-Event-ID

This complements the existing closeSSEStream() for POST streams.

src/server/streamableHttp.ts

@@ -823,7 +823,11 @@ export class StreamableHTTPServerTransport implements Transport {
      * Use this to implement polling behavior for server-initiated notifications.
      */
     closeStandaloneSSEStream(): void {
-        // TODO: implement - currently does nothing, stream won't close
+        const stream = this._streamMapping.get(this._standaloneSseStreamId);
+        if (stream) {
+            stream.end();
+            this._streamMapping.delete(this._standaloneSseStreamId);
+        }
     }
 
     async send(message: JSONRPCMessage, options?: { relatedRequestId?: RequestId }): Promise<void> {
@@ -841,19 +845,21 @@ export class StreamableHTTPServerTransport implements Transport {
             if (isJSONRPCResponse(message) || isJSONRPCError(message)) {
                 throw new Error('Cannot send a response on a standalone SSE stream unless resuming a previous client request');
             }
-            const standaloneSse = this._streamMapping.get(this._standaloneSseStreamId);
-            if (standaloneSse === undefined) {
-                // The spec says the server MAY send messages on the stream, so it's ok to discard if no stream
-                return;
-            }
 
             // Generate and store event ID if event store is provided
+            // Store even if stream is disconnected so events can be replayed on reconnect
             let eventId: string | undefined;
             if (this._eventStore) {
                 // Stores the event and gets the generated event ID
                 eventId = await this._eventStore.storeEvent(this._standaloneSseStreamId, message);
             }
 
+            const standaloneSse = this._streamMapping.get(this._standaloneSseStreamId);
+            if (standaloneSse === undefined) {
+                // Stream is disconnected - event is stored for replay, nothing more to do
+                return;
+            }
+
             // Send the message to the standalone SSE stream
             this.writeSSEEvent(standaloneSse, message, eventId);
             return;