fix: address Copilot review round 6 on #2318

RemoteClientTransport tracks open streams only when onFetchStreamUpdate
is registered — the bookkeeping serves that callback alone, and without
it the set would grow by one entry per reconnect.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01APKSYHGxXBUpnR7ErFiLJn
Signed-off-by: cliffhall <cliff@futurescale.com>

core/mcp/remote/remoteClientTransport.ts

@@ -556,8 +556,11 @@ export class RemoteClientTransport implements Transport {
             const entry = parsed.data;
             // Stream bookkeeping runs whether or not the consumer logs
             // requests: `onFetchStreamUpdate` is documented to work on its
-            // own, and `close()` needs to know which streams are open.
+            // own, and `close()` needs to know which streams are open. It
+            // exists only to serve that callback, though — without one the
+            // set would only ever grow, one entry per reconnect.
             if (
+              this.options.onFetchStreamUpdate &&
               isLongLivedStreamResponse(
                 entry.method,
                 findHeader(entry.responseHeaders, "content-type"),