fix: better isPromise check for proxy objects (#11178)

.changeset/green-moose-fetch.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Improves `isPromise` utility to check the presence of `then` on an object before trying to access it - which can cause undesired side-effects on Proxy objects

packages/astro/src/runtime/server/util.ts

@@ -1,5 +1,5 @@
 export function isPromise<T = any>(value: any): value is Promise<T> {
-	return !!value && typeof value === 'object' && typeof value.then === 'function';
+	return !!value && typeof value === 'object' && 'then' in value && typeof value.then === 'function';
 }
 
 export async function* streamAsyncIterator(stream: ReadableStream) {