fix: match index files only by using entire basename

The previous code would cause file names like `index.xml.ts` to be
treated as index files, when they aren't (the file basename excluding
the extension `.ts` is `index.xml`, and `index` should be the index
file). To match index files only, match the entire first half of the
basename when splitting by the last occurrence of the period/file
extension delimiter.

Pedantically this might not be correct -- i.e. some file extensions
actually span multiple delimiters, like index.tar.gz. But in that case
it's not an index file as well, so I think this is fine.

Fixes #12675

packages/astro/src/core/routing/manifest/create.ts

@@ -183,7 +183,7 @@ function createFileBasedRoutes(
 			validateSegment(segment, file);
 
 			const parts = getParts(segment, file);
-			const isIndex = isDir ? false : basename.startsWith('index.');
+			const isIndex = isDir ? false : basename.substring(0, basename.lastIndexOf('.')) === "index";
 			const routeSuffix = basename.slice(basename.indexOf('.'), -ext.length);
 			const isPage = validPageExtensions.has(ext);