Mark a task as completed (or incomplete) and optionally record the caregiver's response.

Workflow — completing a task with a response value:

1. Fetch the task list with GET /visits/{visitId}/evv/tasks
2. Inspect the task's questionType and options array to determine what response is expected
3. Submit the completion with the response value:
   • For radio / scale / checkbox tasks: pass selectedOptionIds (array of option id
     values from the task's options array). Do not pass value — it is rejected for
     these question types because it would not be displayed in the Viv UI. Use a
     one-element array for radio/scale, multi-element for checkbox.
   • For text tasks: pass value with the free-form text response. Do not pass
     selectedOptionIds.
   • For number tasks: pass value with the numeric response, optionally suffixed with
     a unit from the task's units array (e.g. "30 minutes"). Do not pass
     selectedOptionIds.
   • If no response value is needed, just pass completed: true.

Side effects on completion:

• A client feed item is created (visible in the client's activity feed)
• Admin notifications are triggered
• The visit's cached task count (queries.tasks) is updated

Marking incomplete:
Setting completed: false clears the completion timestamp. The task will need to be
re-submitted by the caregiver.

Error codes (400):

• INVALID_FIELD_FOR_QUESTION_TYPE — value was sent on a radio/scale/checkbox task, or
  selectedOptionIds was sent on a text/number task. The Viv SVE panel and downstream
  exporters render different fields per question type; sending the wrong field would
  silently produce a response that does not appear in the UI.
• INVALID_OPTION_ID — one or more entries in selectedOptionIds are not present on the
  task's options array. Re-fetch the task to get current option ids.