[Bug] Job cost includes time before provisioning #4340

Summary

Job submission cost appears to include elapsed time from submitted_at, even when no instance has been assigned. This can make a queued job display a large cost that is unrelated to its actual compute interval; the value can also change on reads while a submission is active.

Expected behavior

• No job cost accrues before the job is assigned/provisioned on billable compute.
• Cost duration uses the authoritative job provisioning/assignment start and finish timestamps, not queue submission time.
• Reused instances are handled using the job interval rather than charging the instance lifetime to every job.
• Active cost reads are stable and do not retroactively price queue time.

Please add tests for never-provisioned, delayed-provisioning, finished, active, and reused-instance cases.