fix(gateway): preserve cron delivery validation semantics

[obviyus]

Keeps cron delivery validation aligned with actual runtime behavior.

Fixes the two follow-ups from #69015: disabled channels no longer create false ambiguity, and delivery patch validation now uses the live cron service default agent.

[greptile-apps]

Greptile Summary

This PR fixes two validation alignment issues in cron delivery: listConfiguredAnnounceChannelIds now filters out channels where enabled: false so they don't count toward the "multiple configured channels" ambiguity threshold, and assertValidCronUpdateDelivery now forwards the live defaultAgentId from context.cron.getDefaultAgentId() into applyJobPatch so the patch simulation matches actual runtime behavior. Both fixes are backed by new integration tests.

Confidence Score: 5/5

Safe to merge — both fixes are narrow, well-reasoned, and covered by new integration tests.

No P0 or P1 findings. The disabled-channel filter (enabled !== false) correctly treats channels without an explicit enabled field as enabled, the TOCTOU gap between validation and update is an acceptable design trade-off, and the three new tests directly exercise both bug scenarios.

No files require special attention.

_{Reviews (1): Last reviewed commit: "docs(changelog): note cron delivery vali..." | Re-trigger Greptile}

[obviyus]

Landed on main.

• Landed source commit: f4c2c92
• Merge commit: c206702