fix(plugin-rsc): use explicit marker for CJS module detection in interop helper

- Change interop helper to check for __cjs_module_runner_transform marker
- Add marker export to all transformed CJS modules
- Update tests to include marker in transformed modules
- More explicit approach than heuristic-based detection

Co-authored-by: hi-ogawa <4232207+hi-ogawa@users.noreply.github.com>

Author: Copilot

packages/plugin-rsc/src/plugins/cjs.ts

@@ -60,6 +60,7 @@ export function cjsModuleRunnerPlugin(): Plugin[] {
           output.append(`
 ;__vite_ssr_exportAll__(module.exports);
 export default module.exports;
+export const __cjs_module_runner_transform = true;
 `)
           return {
             code: output.toString(),

packages/plugin-rsc/src/transforms/cjs.test.ts

@@ -38,7 +38,7 @@ if (true) {
 `
     expect(await testTransform(input)).toMatchInlineSnapshot(`
       "let exports = {}; const module = { exports };
-      function __cjs_interop__(m) { return m && m.__esModule ? m.default : (m.default !== undefined ? m.default : m); }
+      function __cjs_interop__(m) { return m.__cjs_module_runner_transform ? m.default : m; }
       if (true) {
         module.exports = (__cjs_interop__(await import('./cjs/use-sync-external-store.production.js')));
       } else {
@@ -58,7 +58,7 @@ if (true) {
 `
     expect(await testTransform(input)).toMatchInlineSnapshot(`
       "let exports = {}; const module = { exports };
-      function __cjs_interop__(m) { return m && m.__esModule ? m.default : (m.default !== undefined ? m.default : m); }
+      function __cjs_interop__(m) { return m.__cjs_module_runner_transform ? m.default : m; }
       const __cjs_to_esm_hoist_0 = __cjs_interop__(await import("react"));
       const __cjs_to_esm_hoist_1 = __cjs_interop__(await import("react-dom"));
       "production" !== process.env.NODE_ENV && (function() { 
@@ -84,7 +84,7 @@ function test() {
 `
     expect(await testTransform(input)).toMatchInlineSnapshot(`
       "let exports = {}; const module = { exports };
-      function __cjs_interop__(m) { return m && m.__esModule ? m.default : (m.default !== undefined ? m.default : m); }
+      function __cjs_interop__(m) { return m.__cjs_module_runner_transform ? m.default : m; }
       const __cjs_to_esm_hoist_0 = __cjs_interop__(await import("te" + "st"));
       const __cjs_to_esm_hoist_1 = __cjs_interop__(await import("test"));
       const __cjs_to_esm_hoist_2 = __cjs_interop__(await import("test"));
@@ -133,6 +133,7 @@ function test() {
               output.append(`
 ;__vite_ssr_exportAll__(module.exports);
 export default module.exports;
+export const __cjs_module_runner_transform = true;
 `)
               return {
                 code: output.toString(),
@@ -159,6 +160,7 @@ export default module.exports;
           "value": 3,
         },
         "depNamespace": {
+          "__cjs_module_runner_transform": true,
           "a": "a",
           "b": "b",
           "default": {

packages/plugin-rsc/src/transforms/cjs.ts

@@ -4,11 +4,9 @@ import { analyze } from 'periscopic'
 import { walk } from 'estree-walker'
 
 // Runtime helper to handle CJS/ESM interop when transforming require() to import()
-// This is needed because when CJS code does require("pkg"), it expects:
-// - For CJS modules: the module.exports value directly
-// - For ESM modules: Node.js actually returns the namespace object
-// Since we're transforming to dynamic import(), we need to handle both cases
-const CJS_INTEROP_HELPER = `function __cjs_interop__(m) { return m && m.__esModule ? m.default : (m.default !== undefined ? m.default : m); }`
+// Only unwrap .default for modules that were transformed by this plugin (marked with __cjs_module_runner_transform)
+// This ensures we don't incorrectly unwrap .default on genuine ESM modules
+const CJS_INTEROP_HELPER = `function __cjs_interop__(m) { return m.__cjs_module_runner_transform ? m.default : m; }`
 
 export function transformCjsToEsm(
   code: string,