Optional checkpoint selections return JSON instead of none

Training and checkpoint listing succeeded, but checkpoint review returned:

selected-checkpoint-1: checkpoint-9
selected-checkpoint-2: {"enum": ["none", "checkpoint-9"]}
selected-checkpoint-3: {"enum": ["none", "checkpoint-9"]}

The checkpoint check then failed with:

ValueError: select one to three distinct checkpoints

Reproduced in the Argo Workflows v4.1.1 UI by selecting the first checkpoint and leaving both optional fields untouched before confirming. Both optional fields had `none` as their resolved default.

Selecting one checkpoint should work without touching the optional fields. Those fields should return `none`, not the dropdown options as JSON. Add a regression test for this flow and keep rejecting malformed values and duplicate checkpoints.

[WorkflowTemplate](https://github.com/ai-cfia/nachet/blob/main/mlops/workflows/detector-training-workflow-template.yaml)
[Argo input form](https://github.com/argoproj/argo-workflows/blob/v4.1.1/ui/src/workflows/components/workflow-details/suspend-inputs.tsx)

Related to #893.