fix: Search ranking on merge indexes (#2994)

Co-authored-by: Emilien GUILMINEAU (Ext.) <emilien.guilmineau.prestataire@sfr.com>
Co-authored-by: Chris Swithinbank <swithinbank@gmail.com>

Author: 3 people

.changeset/five-glasses-shout.md

@@ -0,0 +1,5 @@
+---
+"@astrojs/starlight": patch
+---
+
+Fixes default ranking of merged indexes when using multi-site search

packages/starlight/schemas/pagefind.ts

@@ -67,7 +67,8 @@ const pagefindIndexOptionsSchema = z.object({
 	/**
 	 * Configure how search result rankings are calculated by Pagefind.
 	 */
-	ranking: pagefindRankingWeightsSchema.optional(),
+	 // We apply a default value to merged indexes in order to share the same ranking for them and the current site when not set explicitly.
+	ranking: pagefindRankingWeightsSchema.default({}),
 });
 
 const pagefindSchema = z.object({