Motivation for the change, related issues

runCLI() in packages/playground/cli/src/run-cli.ts had a .catch() on the internal startServer() call that swallowed startup errors by printing them and calling process.exit(1):

}).catch((error) => {
    cliOutput.printError(describeError(error));
    process.exit(1);
});

This means library callers of runCLI() — such as WordPress Studio, which spawns it as a daemon — can never catch or handle startup errors. A non-fatal error (e.g. a port conflict, a transient worker exit race) kills the entire process with no opportunity to retry or report it gracefully.

Fixes #3520

Implementation details

Remove the .catch() so rejections from startServer() propagate naturally through await as a rejected promise from runCLI().

The process.exit(1) behavior for standalone CLI users is preserved unchanged: parseOptionsAndRunCLI() already wraps runCLI() in its own try/catch that formats the error and exits.

The existing test should error when explicit port is already in use was asserting the old broken behavior (error printed to stdout + process.exit called). Updated it to assert the correct behavior: runCLI() rejects with EADDRINUSE and process.exit is not called.

Breaking change: library callers that previously relied on runCLI() never rejecting will now see unhandled rejections on startup failure. This is the correct behavior — callers should handle errors from runCLI().

Testing Instructions (or ideally a Blueprint)

1. Run the CLI test suite and confirm all tests pass:

   nvm use
   npx nx test-playground-cli playground-cli --testFile=run-cli.spec.ts

2. Confirm the standalone CLI still exits cleanly on a port conflict:

   # Occupy port 9400
   node -e "require('net').createServer().listen(9400)"
   # In another terminal, try to start the CLI on the same port
   npx nx dev playground-cli server --port=9400
   # Should print the EADDRINUSE error and exit with code 1 — same as before

3. Confirm library callers can now catch errors:

   import { runCLI } from '@wp-playground/cli';
   
   try {
       await runCLI({ command: 'server', port: 9400 }); // port already in use
   } catch (error) {
       console.log('Caught:', error.message); // now reachable
   }