fix: guard the abort listener against a double invocation

Copilot review round 1. The helper's contract is exactly-once teardown
and the test asserted it must not lean on Hono's own re-entry guard, but
neither the implementation nor the test actually enforced that: the
listener had no local guard and the test aborted only once.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/test/integration/mcp/remote/sse-abort-cleanup.test.ts

@@ -138,9 +138,12 @@ describe("primeAndHoldSseStream (#1999)", () => {
 
     s.finishWrite();
     s.abort();
+    // Hono guards re-entry itself, but the helper must not depend on that —
+    // so drive the listener a second time directly.
+    s.abort();
     await withinTick(held);
-    // Hono guards re-entry itself, but the helper must not depend on that.
     expect(cleanups).toBe(1);
+    expect(s.closes).toBe(1);
   });
 
   it("does not reject when cleanup's own close loses the race", async () => {

core/mcp/remote/node/server.ts

@@ -147,8 +147,15 @@ export async function primeAndHoldSseStream(
   stream: PrimableSseStream,
   cleanup: () => void,
 ): Promise<void> {
+  let done = false;
   const aborted = new Promise<void>((resolve) => {
     stream.onAbort(() => {
+      // Hono's `abort()` guards its own re-entry, but the guard belongs here
+      // too: this helper's contract is exactly-once teardown, and running a
+      // caller's cleanup twice would double-delete a subscriber that has
+      // since been re-added by a reconnect.
+      if (done) return;
+      done = true;
       cleanup();
       closeAbortedStream(stream);
       resolve();