fix: workaround noExternal merging bug on Vite 6 (#8950)

Author: hi-ogawa

packages/vitest/src/node/plugins/runnerTransform.ts

@@ -145,6 +145,12 @@ export function ModuleRunnerTransform(): VitePlugin {
           // in both SSR and Client environments
           environment.resolve.noExternal = true
 
+          // Workaround `noExternal` merging bug on Vite 6
+          // https://github.com/vitejs/vite/pull/20502
+          if (name === 'ssr') {
+            delete config.ssr?.noExternal
+          }
+
           if (name === '__vitest_vm__' || name === '__vitest__') {
             continue
           }