* fix(cron): accept null fallbacks in cron update patch payload
createCronPatchObjectSchema passes nullableStringArraySchema for model and
toolsAllow, but fallbacks was still hardcoded as a non-nullable array in
cronPayloadObjectSchema, rejecting null clears before they reach the
Gateway service layer.
Switch fallbacks to a parameter so the patch context can pass the nullable
variant, matching the existing pattern for model and toolsAllow.
Fixes#100707
Co-Authored-By: Claude <noreply@anthropic.com>
* test(cron): cover model+fallbacks null clear in patch schema test
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Co-authored-by: Claude <noreply@anthropic.com>