Replace copy/pasted ImageFunction with imported SchemaContext (#775)

* Replace copy/pasted `ImageFunction` with imported `SchemaContext`

* Only import type from astro:content

* Add changeset

* Improve changeset

.changeset/fair-trains-care.md

@@ -0,0 +1,5 @@
+---
+"@astrojs/starlight": patch
+---
+
+Fix content collection schema compatibility with Astro 3.1 and higher

packages/starlight/schema.ts

@@ -1,4 +1,5 @@
 import { z } from 'astro/zod';
+import type { SchemaContext } from 'astro:content';
 import { HeadConfigSchema } from './schemas/head';
 import { PrevNextLinkConfigSchema } from './schemas/prevNextLink';
 import { TableOfContentsSchema } from './schemas/tableOfContents';
@@ -9,25 +10,8 @@ export { i18nSchema } from './schemas/i18n';
 type IconName = keyof typeof Icons;
 const iconNames = Object.keys(Icons) as [IconName, ...IconName[]];
 
-type ImageFunction = () => z.ZodObject<{
-	src: z.ZodString;
-	width: z.ZodNumber;
-	height: z.ZodNumber;
-	format: z.ZodUnion<
-		[
-			z.ZodLiteral<'png'>,
-			z.ZodLiteral<'jpg'>,
-			z.ZodLiteral<'jpeg'>,
-			z.ZodLiteral<'tiff'>,
-			z.ZodLiteral<'webp'>,
-			z.ZodLiteral<'gif'>,
-			z.ZodLiteral<'svg'>,
-		]
-	>;
-}>;
-
 export function docsSchema() {
-	return ({ image }: { image: ImageFunction }) =>
+	return ({ image }: SchemaContext) =>
 		z.object({
 			/** The title of the current page. Required. */
 			title: z.string(),