fix(ext/node): wrap non-Error unhandled rejections in ERR_UNHANDLED_REJECTION (#32535)

## Summary

- When a promise is rejected with a non-Error value (e.g.
`Promise.reject(null)`) and there are no `unhandledRejection` listeners,
Node.js wraps the rejection reason in an `ERR_UNHANDLED_REJECTION` error
before routing it to `uncaughtException`. Deno was passing the raw value
directly, which caused crashes when exception handlers accessed
`.message` or `.name`.
- This fixes `test-promise-unhandled-default.js` in the node compat test
suite (also unblocks `test-promise-unhandled-throw.js` which tests the
same wrapping logic under `--unhandled-rejections=throw`).

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: bartlomieju

ext/node/polyfills/process.ts

@@ -1086,7 +1086,29 @@ function synchronizeListeners() {
         // listeners.
 
         event.preventDefault();
-        uncaughtExceptionHandler(event.reason, "unhandledRejection");
+
+        let reason = event.reason;
+        // If the rejection reason is not an Error, wrap it in an
+        // ERR_UNHANDLED_REJECTION error, matching Node.js behavior.
+        if (!(reason instanceof Error)) {
+          const message = "This error originated either by throwing " +
+            "inside of an async function without a catch block, or by rejecting a " +
+            "promise which was not handled with .catch(). The promise rejected with the" +
+            ` reason "${reason}".`;
+          const err = new Error(message);
+          // deno-lint-ignore no-explicit-any
+          (err as any).code = "ERR_UNHANDLED_REJECTION";
+          // deno-lint-ignore no-explicit-any
+          (err as any).reason = event.reason;
+          Object.defineProperty(err, "name", {
+            value: "UnhandledPromiseRejection",
+            writable: true,
+            configurable: true,
+          });
+          reason = err;
+        }
+
+        uncaughtExceptionHandler(reason, "unhandledRejection");
         return;
       }