Playground hides managed-runtime readiness diagnostics

[rmcdaniel]

User-visible failure

When the managed-runtime worker roster command fails, scripts/playground reports only that dw worker:list exited with status 8. The CLI's actionable compatibility diagnostic is captured but discarded, forcing a first-time user to reconstruct and rerun the internal command manually.

Required outcome

• Preserve the worker-roster readiness proof; do not silently bypass compatibility or registration checks.
• Include the failed command, exit status, and sanitized captured stderr/stdout in the playground error.
• Add focused regression coverage for failed JSON commands and the final readiness timeout message.
• Verify the managed PHP playground reports a useful failure against an incompatible fixture and succeeds against the corrected stable runtime.

The Server compatibility defect is tracked separately in durable-workflow/server#89.

[rmcdaniel]

Verified in PR #76: the managed playground now preserves the failed readiness command output and its last diagnostic, so compatibility and registration failures are actionable instead of collapsing to an exit code. All required checks passed and the fix is merged on main.