fix(server): prevent duplicate and accidental keepalive timer clears

Adds defensive _clearKeepAliveTimer() before setInterval in
handleGetRequest() to prevent duplicate timers when the GET stream is
reinitialized.

Moves _clearKeepAliveTimer() inside the if(stream) guard in
closeStandaloneSSEStream() so it only clears when the standalone stream
is being torn down, preventing accidental clearing of a replay stream's
timer.

Closes #1726

Author: rcdailey

packages/server/src/server/streamableHttp.ts

@@ -490,6 +490,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
         // Start keepalive timer to send periodic SSE comments that prevent
         // reverse proxies from closing the connection due to idle timeouts
         if (this._keepAliveInterval !== undefined) {
+            this._clearKeepAliveTimer();
             this._keepAliveTimer = setInterval(() => {
                 try {
                     streamController!.enqueue(encoder.encode(': keepalive\n\n'));
@@ -977,9 +978,9 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
      * Use this to implement polling behavior for server-initiated notifications.
      */
     closeStandaloneSSEStream(): void {
-        this._clearKeepAliveTimer();
         const stream = this._streamMapping.get(this._standaloneSseStreamId);
         if (stream) {
+            this._clearKeepAliveTimer();
             stream.cleanup();
         }
     }