Fixes openclaw#652

When cron jobs with sessionTarget:"main" have wakeMode:"now",
they were being marked as completed immediately without waiting for the
agent to actually process the system event.

The issue was that requestHeartbeatNow() is fire-and-forget and
doesn't wait for the heartbeat to complete. The job would finish
with durationMs: 0 before the agent had a chance to run.

This fix:
- Adds runHeartbeatOnce to CronServiceDeps
- Wires it up in gateway/server.ts to load config and pass runtime
- Modifies executeJob() to call runHeartbeatOnce when wakeMode:"now"
- Waits for heartbeat to complete and maps status to cron result:
  * "ran" → "ok"
  * "skipped" → "skipped"
  * "failed" → "error"
- Falls back to old behavior for wakeMode:"next-heartbeat" or if
  runHeartbeatOnce is not available (backward compatibility)

Benefits:
- Jobs now have accurate durationMs reflecting actual processing time
- Jobs are correctly marked with "error" status if heartbeat fails
- Prevents race condition where job completes before agent runs

[AI-assisted] - Generated with z.ai GLM-4.7
[Tested: Lightly tested - Logic validated with test scenarios, code quality checks passed, integration testing requires live Clawdbot instance]