fix: preserve automation oncall update fields

[ysyneu]

Summary

• add on-call incident trigger fields to AutomationRuleUpdateRequest
• preserve nil vs false/empty partial-update semantics for update requests
• add JSON round-trip regression coverage for the new fields

Verification

• make check
• go test ./... -run 'TestAutomationRuleUpdateRequestCanSendFalseValues|TestAutomationRuleUpdateRequestPreservesOncallIncidentTriggerFields' -count=1
• go generate ./... && git diff --exit-code --stat