fix: access of undefined in runtime that does have great support of instanceof (#1838)

* fix: access of undefined in open-nextjs hosted runtime

- This change follows the "duck typing" principle which is often more robust than strict instance checking, especially in complex JavaScript environments where objects might come from different contexts or realms.
- Fixes issue where in an opennext hosted runtime, instanceof usage is not sufficent.

* feat: use in operator for better reliabiltiy across runtimes

* fix: linting issue

packages/better-auth/src/cookies/index.ts

@@ -249,7 +249,7 @@ export const getSessionCookie = (
 			config.cookiePrefix = `${config.cookiePrefix}.`;
 		}
 	}
-	const headers = request instanceof Headers ? request : request.headers;
+	const headers = "headers" in request ? request.headers : request;
 	const req = request instanceof Request ? request : undefined;
 	const url = getBaseURL(req?.url, config?.path, req);
 	const cookies = headers.get("cookie");

packages/better-auth/src/utils/get-request-ip.ts

@@ -24,7 +24,7 @@ export function getIp(
 		"forwarded-for",
 		"forwarded",
 	];
-	const headers = req instanceof Request ? req.headers : req;
+	const headers = "headers" in req ? req.headers : req;
 	for (const key of keys) {
 		const value = headers.get(key);
 		if (typeof value === "string") {