Is your feature request related to a problem? Please describe.

The new task/list method's specification dictates that tasks must be ordered by the last update time. However, the current data model lacks a direct way to determine when a task was last updated.

Describe the solution you'd like

To resolve this, a new field with the last update time is being added to the Task message.

Describe alternatives you've considered

1. The timestamp field in the TaskStatus message already exists: There are two issues with this field:
   1. Not all task updates are necessarily status updates.
   2. The field is optional, which can lead to unstable ordering when sorting by this field alone.
2. Backends can just incorporate this information into their internal models: While true, including this in the public API offers significant advantages by allowing clients to readily access and display this value to users.

Additional context

No response

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