fix: merge req.body modifications into decoded request

Added test case to verify that middleware modifications to the req
object persist through the middleware chain. This addresses the issue
where normal Express middleware behavior wasn't being tested with a
realistic use case.

Author: yanxue-22

packages/express-wrapper/src/middleware.ts

@@ -216,5 +216,5 @@ export async function runMiddlewareChainIgnoringResults<
       }
     });
   }
-  return input;
+  return { ...input, ...(req.body || {}) };
 }

packages/express-wrapper/test/middleware.test.ts

@@ -40,6 +40,38 @@ test('should handle errors passed to next()', async () => {
   );
 });
 
+test('middleware that modifies req should pass changes through chain', async () => {
+  const testReq = { body: {}, headers: {} } as express.Request;
+  const testRes = {} as express.Response;
+
+  const modifyReqMiddleware: express.RequestHandler = (req, _res, next) => {
+    (req as any).userId = 123;
+    next();
+  };
+
+  const checkReqMiddleware: express.RequestHandler = (req, _res, next) => {
+    assert.equal(
+      (req as any).userId,
+      123,
+      'userId should still be on req in second middleware',
+    );
+    next();
+  };
+
+  await runMiddlewareChain(
+    { foo: 'test' },
+    [modifyReqMiddleware, noopMiddleware, checkReqMiddleware],
+    testReq,
+    testRes,
+  );
+
+  assert.equal(
+    (testReq as any).userId,
+    123,
+    'req should still have userId after chain',
+  );
+});
+
 test('should work with middleware that return values', async () => {
   const result = await runMiddlewareChain(
     { foo: 'test' },