test(cron): add model fallback regression coverage

CHANGELOG.md

@@ -40,6 +40,7 @@ Docs: https://docs.openclaw.ai
 - Slack: restrict forwarded-attachment ingestion to explicit shared-message attachments and skip non-Slack forwarded `image_url` fetches, preventing non-forward attachment unfurls from polluting inbound agent context while preserving forwarded message handling.
 - Feishu: detect bot mentions in post messages with embedded docs when `message.mentions` is empty. (#18074) Thanks @popomore.
 - Agents/Sessions: align session lock watchdog hold windows with run and compaction timeout budgets (plus grace), preventing valid long-running turns from being force-unlocked mid-run while still recovering hung lock owners. (#18060)
+- Cron: preserve default model fallbacks for cron agent runs when only `model.primary` is overridden, so failover still follows configured fallbacks unless explicitly cleared with `fallbacks: []`. (#18210) Thanks @mahsumaktas.
 - Cron/Heartbeat: canonicalize session-scoped reminder `sessionKey` routing and preserve explicit flat `sessionKey` cron tool inputs, preventing enqueue/wake namespace drift for session-targeted reminders. (#18637) Thanks @vignesh07.
 - Cron/Webhooks: reuse existing session IDs for webhook/cron runs when the session key is stable and still fresh, preserving conversation history. (#18031) Thanks @Operative-001.
 - Cron: prevent spin loops when cron jobs complete within the scheduled second by advancing the next run and enforcing a minimum refire gap. (#18073) Thanks @widingmarcus-cyber.