Refactor ondelete hook to onsessionclosed in StreamableHTTPServerTransport

- Renamed ondelete callback to onsessionclosed for better clarity
- Moved callback from public property to private field following options pattern
- Added comprehensive JSDoc documentation explaining the callback's purpose
- Updated DELETE request handler to use the new callback structure

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/server/streamableHttp.ts

@@ -49,6 +49,18 @@ export interface StreamableHTTPServerTransportOptions {
    */
   onsessioninitialized?: (sessionId: string) => void;
 
+  /**
+   * A callback for session close events
+   * This is called when the server closes a session due to a DELETE request.
+   * Useful in cases when you need to clean up resources associated with the session.
+   * Note that this is different from the transport closing, if you are handling 
+   * HTTP requests from multiple nodes you might want to close each 
+   * StreamableHTTPServerTransport after a request is completed while still keeping the 
+   * session open/running.
+   * @param sessionId The session ID that was closed
+  */
+  onsessionclosed?: (sessionId: string) => void;
+
   /**
    * If true, the server will return JSON responses instead of starting an SSE stream.
    * This can be useful for simple request/response scenarios without streaming.
@@ -127,13 +139,13 @@ export class StreamableHTTPServerTransport implements Transport {
   private _standaloneSseStreamId: string = '_GET_stream';
   private _eventStore?: EventStore;
   private _onsessioninitialized?: (sessionId: string) => void;
+  private _onsessionclosed?: (sessionId: string) => void;
   private _allowedHosts?: string[];
   private _allowedOrigins?: string[];
   private _enableDnsRebindingProtection: boolean;
 
   sessionId?: string;
   onclose?: () => void;
-  ondelete?: (sessionId: string) => void;
   onerror?: (error: Error) => void;
   onmessage?: (message: JSONRPCMessage, extra?: MessageExtraInfo) => void;
 
@@ -142,6 +154,7 @@ export class StreamableHTTPServerTransport implements Transport {
     this._enableJsonResponse = options.enableJsonResponse ?? false;
     this._eventStore = options.eventStore;
     this._onsessioninitialized = options.onsessioninitialized;
+    this._onsessionclosed = options.onsessionclosed;
     this._allowedHosts = options.allowedHosts;
     this._allowedOrigins = options.allowedOrigins;
     this._enableDnsRebindingProtection = options.enableDnsRebindingProtection ?? false;
@@ -539,7 +552,7 @@ export class StreamableHTTPServerTransport implements Transport {
     if (!this.validateProtocolVersion(req, res)) {
       return;
     }
-    this.ondelete?.(this.sessionId!);
+    this._onsessionclosed?.(this.sessionId!);
     await this.close();
     res.writeHead(200).end();
   }