fix(cli): resolve port conflicts in parallel test execution (#11363)

* fix(cli): resolve port conflicts in parallel test execution

Fixes EADDRINUSE errors when running rspack-cli tests in parallel by:

1. Auto-assigning random ports to serve commands that don't specify --port
2. Using Jest worker ID to ensure unique port ranges per worker (similar to e2e tests)
3. Preventing multiple test workers from competing for default port 8080

This resolves flaky test failures in tests/serve/basic/serve-basic.test.ts
and other serve tests when run concurrently.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

* docs: improve getRandomPort function documentation

Add comprehensive JSDoc explaining the port allocation strategy
and how it prevents EADDRINUSE conflicts in parallel test execution.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

---------

Co-authored-by: Claude <[email protected]>

Author: h-a-n-a

packages/rspack-cli/tests/utils/test-utils.ts

@@ -114,7 +114,16 @@ const runWatch = (
 	options: Record<string, any> = {},
 	env: Record<string, any> = {}
 ): any => {
-	return new Promise((resolve, reject) => {
+	return new Promise(async (resolve, reject) => {
+		// For serve commands, automatically add random port if no --port specified
+		if (args.includes("serve") && !args.some(arg => arg.startsWith("--port"))) {
+			const randomPort = await getRandomPort();
+			args.push(`--port=${randomPort}`);
+			console.log(
+				`[runWatch] Auto-assigned random port ${randomPort} for serve command`
+			);
+		}
+
 		const process = createProcess(cwd, args, options, env);
 		const outputKillStr = options.killString || /rspack \d+\.\d+\.\d/;
 
@@ -411,13 +420,26 @@ function isPortAvailable(port: number) {
 
 const portMap = new Map();
 
-// Available port ranges: 1024 ～ 65535
-// `10080` is not available in macOS CI, `> 50000` get 'permission denied' in Windows.
-// so we use `15000` ~ `45000`.
-export async function getRandomPort(
-	defaultPort = Math.ceil(Math.random() * 30000) + 15000
-) {
-	let port = defaultPort;
+/**
+ * Get a random available port for testing.
+ *
+ * Uses Jest worker ID to assign unique base ports per worker to prevent conflicts
+ * in parallel test execution. Similar to e2e tests approach:
+ * - Worker 1 gets ports starting from 15000
+ * - Worker 2 gets ports starting from 15100
+ * - Worker 3 gets ports starting from 15200
+ * - etc.
+ *
+ * Available port ranges: 1024 ～ 65535
+ * `10080` is not available in macOS CI, `> 50000` get 'permission denied' in Windows.
+ * So we use `15000` ~ `45000`.
+ */
+export async function getRandomPort(defaultPort?: number) {
+	// Use Jest workerIndex to assign unique base ports, similar to e2e tests
+	const workerIndex = parseInt(process.env.JEST_WORKER_ID || "1", 10);
+	const basePort = defaultPort || 15000 + (workerIndex - 1) * 100;
+
+	let port = basePort;
 	while (true) {
 		if (!portMap.get(port) && (await isPortAvailable(port))) {
 			portMap.set(port, 1);