Summary:
Test was failing because it takes some time for old jobs to get canceled after leader change.
Fixed by sleeping for this duration.

Also added some extra safety in `CanStartTask` to check for leader mode.

Fixes #25791
Jira: DB-15079

Test Plan: PgCronTest.CancelJobOnLeaderChange

Reviewers: sergei

Reviewed By: sergei

Subscribers: svc_phabricator, ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D41526