refactor: simplify close() guard to match stdio.ts pattern

Use a _closed boolean flag instead of _closing, remove the snapshot
and try/catch per review feedback. Matches the existing pattern in
server/stdio.ts. Both tests still pass with this simpler approach.

Author: claygeo

packages/server/src/server/streamableHttp.ts

@@ -899,34 +899,22 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
         return undefined;
     }
 
-    private _closing = false;
+    private _closed = false;
 
     async close(): Promise<void> {
-        // Guard against re-entrant calls. When onclose() triggers the
-        // Protocol layer to call close() again, this prevents infinite
-        // recursion that causes a stack overflow with many transports.
-        if (this._closing) {
+        if (this._closed) {
             return;
         }
-        this._closing = true;
+        this._closed = true;
 
-        // Snapshot and clear before iterating to avoid issues with
-        // cleanup callbacks that modify the map during iteration.
-        const streams = [...this._streamMapping.values()];
-        this._streamMapping.clear();
-        this._requestResponseMap.clear();
-
-        // Close all SSE connections with error isolation so one
-        // failing cleanup doesn't prevent others from running.
-        for (const { cleanup } of streams) {
-            try {
-                cleanup();
-            } catch {
-                // Individual stream cleanup failures should not
-                // prevent other streams from being cleaned up.
-            }
+        // Close all SSE connections
+        for (const { cleanup } of this._streamMapping.values()) {
+            cleanup();
         }
+        this._streamMapping.clear();
 
+        // Clear any pending responses
+        this._requestResponseMap.clear();
         this.onclose?.();
     }