Fix async runner response accounting

[chubes4]

Summary

Count only successful HTTP responses as accepted Action Scheduler loopback dispatches. Protected origins can return 403 or 5xx without executing the queue runner; those responses must return zero so callers can use another server-side worker path.

How to test

1. Run php tests/workflow-async-runner-dispatch-result-smoke.php.
2. Verify transport failures report 0.
3. Verify mixed transport failures plus 200/204 report 2.
4. Verify 403/503 report 0.

Closes #453.

AI assistance

• AI assistance: Yes
• Tool: OpenCode
• Used for: Diagnosed a protected-runtime dispatch mismatch and added focused response-accounting coverage. Chris reviews and owns the change.