Fix run_json failure handling for nonzero exits

[abhinavgautam01]

Fixes #1

Summary

Fixes runners.run_json so subprocess failures are always returned as ok: false, even when the failed command emits valid JSON on stdout.

Previously, run_json only returned command_failed when the command exited nonzero and stdout was empty. If a command exited with a nonzero status but printed valid JSON, the helper parsed it and returned ok: true.

Changes

• Treat any nonzero subprocess exit code as command_failed
• Preserve bounded stdout/stderr context in the error detail
• Add a regression test for nonzero exit plus valid JSON stdout

Validation

• PYTHONPATH=src pytest -q
• python3 -m compileall -q src tests
• git diff --check
• CI guard denylist scan
• CI secret-pattern scan