refactor: simplify browser provider peer dep logic and version patch

- Inline peer dep addition into the removal loop, eliminating
  intermediate array and second loop
- Use direct mutation instead of spread for consistency
- Single-pass regex replace in build.ts version patch

Author: fengmk2

packages/cli/src/migration/migrator.ts

@@ -1209,7 +1209,6 @@ export function rewritePackageJson(
     }
   }
   // remove packages that are replaced with vite-plus
-  const removedProviderPackages: string[] = [];
   for (const name of REMOVE_PACKAGES) {
     const wasInDevDeps = !!pkg.devDependencies?.[name];
     const wasInDeps = !!pkg.dependencies?.[name];
@@ -1221,19 +1220,16 @@ export function rewritePackageJson(
       delete pkg.dependencies![name];
       needVitePlus = true;
     }
-    if ((wasInDevDeps || wasInDeps) && BROWSER_PROVIDER_PEER_DEPS[name]) {
-      removedProviderPackages.push(name);
-    }
-  }
-  // Preserve runtime peer dependencies of removed browser provider packages.
-  // e.g., removing @vitest/browser-playwright should keep `playwright` in devDeps.
-  for (const name of removedProviderPackages) {
+    // e.g., removing @vitest/browser-playwright should keep `playwright` in devDeps
     const peerDep = BROWSER_PROVIDER_PEER_DEPS[name];
-    if (peerDep && !pkg.devDependencies?.[peerDep] && !pkg.dependencies?.[peerDep]) {
-      pkg.devDependencies = {
-        ...pkg.devDependencies,
-        [peerDep]: '*',
-      };
+    if (
+      (wasInDevDeps || wasInDeps) &&
+      peerDep &&
+      !pkg.devDependencies?.[peerDep] &&
+      !pkg.dependencies?.[peerDep]
+    ) {
+      pkg.devDependencies ??= {};
+      pkg.devDependencies[peerDep] = '*';
     }
   }
   if (needVitePlus) {

packages/test/build.ts

@@ -1637,19 +1637,17 @@ async function patchVitestBrowserPackage() {
     );
   }
 
-  // 5. Version: var version = "<semver>" → use VP_VERSION env var with fallback
-  // This prevents the "Running mixed versions" warning when VP_VERSION differs
-  // from the bundled vitest browser version.
+  // 5. Patch version to use VP_VERSION, preventing the "Running mixed versions" warning
   const versionPattern = /var version = "(\d+\.\d+\.\d+[^"]*)"/;
-  if (versionPattern.test(content)) {
-    content = content.replace(versionPattern, 'var version = process.env.VP_VERSION || "$1"');
-    console.log('  Patched version to use VP_VERSION env var');
-  } else {
+  const beforeVersion = content;
+  content = content.replace(versionPattern, 'var version = process.env.VP_VERSION || "$1"');
+  if (content === beforeVersion) {
     throw new Error(
       'Failed to patch version in @vitest/browser/index.js: pattern not found. ' +
         'This likely means vitest code has changed and the patch needs to be updated.',
     );
   }
+  console.log('  Patched version to use VP_VERSION env var');
 
   await writeFile(browserIndexPath, content, 'utf-8');
   console.log('  Successfully patched @vitest/browser/index.js');