fix: ensure consistent CSS ordering between dev and build with Turbopack

Fixes #86704

The issue was that CSS files were collected from webpack entrypoints
without sorting, leading to non-deterministic ordering between
development (next dev --turbopack) and production (next build --turbopack).

This commit adds .sort() to the CSS files array in flight-manifest-plugin.ts
to ensure consistent alphabetical ordering in both modes.

The fix ensures that CSS prioritization is deterministic and matches
between dev and build, preventing layout breakage in production.

Author: harshcoder-harsh

packages/next/src/build/webpack/plugins/flight-manifest-plugin.ts

@@ -297,6 +297,7 @@ export class ClientReferenceManifestPlugin {
       manifest.entryCSSFiles[chunkEntryName] = entrypoint
         .getFiles()
         .filter((f) => !f.startsWith('static/css/pages/') && f.endsWith('.css'))
+        .sort() // Sort CSS files to ensure consistent order between dev and build
         .map((file) => {
           const source = compilation.getAsset(file)!.source.source()
           if (