keyvisjob: key visualizer job should set MarkIdle to true

The keyviz job is a forever-running, background job that should not prevent serverless from winding down the SQL pod even when the job is running. This is indicated to the jobs registry by marking the job as idle, see https://github.com/cockroachdb/cockroach/blob/2f811420a1492ca547659003f193233341b7fd87/pkg/spanconfig/spanconfigjob/job.go#L77 as an example.

Jira issue: CRDB-27000