fix: remove unused browser-only PromiseRejectionEvent handler in test

The handler variable using PromiseRejectionEvent was dead code — the
test already uses the Node.js process.on('unhandledRejection') handler.
Removing it fixes the TS2304 type error in CI.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: EtanHey

packages/core/test/shared/protocol.test.ts

@@ -238,12 +238,6 @@ describe('protocol tests', () => {
 
         // Track unhandled rejections
         const unhandledRejections: unknown[] = [];
-        const handler = (event: PromiseRejectionEvent) => {
-            unhandledRejections.push(event.reason);
-            event.preventDefault();
-        };
-
-        // In Node.js, listen on process instead of globalThis
         const processHandler = (reason: unknown) => {
             unhandledRejections.push(reason);
         };