Fix assetsDir in entryFileNames with Vite Env API

Author: markdalgleish

.changeset/rude-cobras-warn.md

@@ -0,0 +1,5 @@
+---
+"@react-router/dev": patch
+---
+
+When `future.unstable_viteEnvironmentApi` is enabled, ensure that `build.assetsDir` in Vite config is respected when `environments.client.build.assetsDir` is not configured

packages/react-router-dev/vite/plugin.ts

@@ -3541,10 +3541,14 @@ export async function getEnvironmentOptionsResolvers(
                 let routeChunkSuffix = routeChunkName
                   ? `-${kebabCase(routeChunkName)}`
                   : "";
-                return path.posix.join(
+                let assetsDir =
                   (ctx.reactRouterConfig.future.unstable_viteEnvironmentApi
                     ? viteUserConfig?.environments?.client?.build?.assetsDir
-                    : viteUserConfig?.build?.assetsDir) ?? "assets",
+                    : null) ??
+                  viteUserConfig?.build?.assetsDir ??
+                  "assets";
+                return path.posix.join(
+                  assetsDir,
                   `[name]${routeChunkSuffix}-[hash].js`
                 );
               },