fix: System.run treated any stderr output as a fatal error

[frostebite]

Fourth bug in the same chain as #79/#81/#82, found by re-running unity-activate#111's CI against v0.1.5.

The bug

System.run rejected on any non-empty stderr, regardless of exit code:

if (runResult.error !== '') {
  reject(new Error(errorMessage));
  return;
}

docker run writes Unable to find image '...' locally to stderr as pure status output when it needs to auto-pull an image — then pulls it and exits 0. The log showed exactly this: Digest: ... / Status: Downloaded newer image for ... (a successful pull) followed immediately by [ERROR] Error: Unable to find image ... locally — the run had already succeeded by the time the error was thrown.

The fix

Check the exit code instead. stderr content is still included in the thrown error message when a command genuinely fails (non-zero exit).

How I found it

v0.1.5 (with #81/#82) got unity-activate#111 past engine detection, target-platform validation, and the broken image tag — this is what showed up next. unityci/editor:ubuntu-2019.2.17f1-base-3 pulled successfully (visible in the log) right before the run was thrown as failed anyway.

Testing

• New tests in system.test.ts: succeeds when stderr has output but exit code is 0; throws on non-zero exit even with empty stderr; throws with stderr content included when a command genuinely fails.
• bun run test — 151 pass, 3 skip, 0 fail.

🤖 Generated with Claude Code

[coderabbitai]

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