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.

Reproduce the Argo Workflows v4.1.1 UI flow using mlops/workflows/detector-training-workflow-template.yaml, selecting one checkpoint and leaving the optional fields untouched. Trace the checkpoint review and validation path, add the requested regression test, and verify that optional values resolve to none while malformed values and duplicate checkpoints remain rejected.

Escrita pelo modelo de indexação a partir do texto da issue.