Update fields for a visit task. All fields are optional; only provided fields are
applied. Setting isCompleted to true records the current server time as the
completion time.

Setting isCompleted to false is a full reset. Every marked.* response
field is cleared (markedValue, normalizedValue, selectedOptionIds,
otherNotes, completedAt) and any of those fields supplied in the same
payload is intentionally ignored — the task will need to be re-submitted by
the caregiver. Top-level note and units are orthogonal to completion
and still applied if present.

Per question-type contract enforced on the canonical PUT:

• radio / checkbox / scale: send selectedOptionIds only. Sending
  markedValue on a SELECT type returns 400 INVALID_FIELD_FOR_QUESTION_TYPE.
  Unknown option ids return 400 INVALID_OPTION_ID.
• text / number: send markedValue only. Sending selectedOptionIds
  on a VALUE type returns 400 INVALID_FIELD_FOR_QUESTION_TYPE.
• number with allowNA: true: send {na: true} to record an
  N/A response when no numeric value applies.
• Report (task.t === 'Report'): parent-level completion requires
  every sub-task to have a response, otherwise 400 REPORT_SUBTASKS_INCOMPLETE.
  Per-subtask completion is not yet exposed on the public API.
• plain Task (no questionType): completion alone is the answer;
  {isCompleted: true} with no value is accepted.

Sending {isCompleted: true} on a question-type task with no response
returns 400 MISSING_RESPONSE_FOR_COMPLETION.