Switch task disablement to the aggregator's new deactivate_at method

In https://github.com/divviup/janus/pull/4684/commits/aacea1292a8bad731428b28b86542b6f28001a44, part of https://github.com/divviup/janus/pull/4684, we're preparing the task's validity interval (task_start + task_duration) to become part of the HPKE-AAD TaskConfiguration. Once that lands, the interval must be immutable -- mutating it would change the AAD and break already-uploaded reports.

As a result, the aggregator's PATCH /tasks/{task_id} endpoint will change:

• The old task_end field (which recomputed the validity interval's duration) goes away.
• It's replaced by deactivate_at, an Janus-specific field that is not part of the TaskConfiguration/AAD, so it can be changed freely at any time.
• Once the aggregator's clock reaches deactivate_at, the task stops accepting report uploads (and helper aggregation), which is the "disable/expire this task" behavior.
• deactivate_at is an ISO 8601 timestamp (e.g. "2026-07-01T00:00:00Z").

We need to:

• Update the aggregator client's task-patch request to send deactivate_at instead of task_end.
• Map the control-plane "disable/expire task" action onto deactivate_at rather than task_expiration. Since both aggregators are provisioned from a single ProvisionableTask, send the same value to leader and helper.
• Confirm nothing else relies on PATCHing the task's validity interval: that interval is now fixed at creation and no longer mutable!