Workflows API broke for CronWorkflows on AW<3.6 using Hera 5.19

[elliotgunton]

Pre-bug-report checklist

1. This bug can be reproduced using pure Argo YAML

• Yes 👉 Please report a bug to the Argo Workflows GitHub 👈
• No

If yes, it is more likely to be an Argo bug unrelated to Hera. Please double check before submitting an issue to Hera.

2. I have searched for existing issues

• Yes

3. This bug occurs in Hera when...

• exporting to YAML
• submitting to Argo
• running on Argo with the Hera runner
• other: using the Workflows client

Bug report

Describe the bug
A clear and concise description of what the bug is:

Error log if applicable:

  File "/app/main.py", line 121, in cron
    cron_workflow.create()
    ~~~~~~~~~~~~~~~~~~~~^^
  File "/app/.venv/lib/python3.13/site-packages/hera/workflows/cron_workflow.py", line 99, in create
    wf = self.workflows_service.create_cron_workflow(
        CreateCronWorkflowRequest(cron_workflow=self.build()),  # type: ignore
        namespace=self.namespace,
    )
  File "/app/.venv/lib/python3.13/site-packages/hera/workflows/service.py", line 509, in create_cron_workflow
    return CronWorkflow(**resp.json())
  File "/app/.venv/lib/python3.13/site-packages/pydantic/v1/main.py", line 341, in __init__
    raise validation_error
pydantic.v1.error_wrappers.ValidationError: 3 validation errors for CronWorkflow
status -> failed
  field required (type=value_error.missing)
status -> phase
  field required (type=value_error.missing)
status -> succeeded
  field required (type=value_error.missing)

To Reproduce

• Install Argo Workflows < 3.6
• Install hera==5.19
• Try to create a CronWorkflow using the create method, with a valid WorkflowsService
• The response will fail to decode

Full Hera code to reproduce the bug:

from hera.workflows import Container, CronWorkflow, WorkflowsService

with CronWorkflow(
    name="hello-world",
    entrypoint="hello-world-with-time",
    schedule="* * * * *",
    timezone="America/Los_Angeles",
    starting_deadline_seconds=0,
    concurrency_policy="Replace",
    successful_jobs_history_limit=4,
    failed_jobs_history_limit=4,
    cron_suspend=False,
    workflows_service=WorkflowsService(
        host="https://localhost:2746",
        namespace="argo",
        verify_ssl=False,
    ),
) as w:
    print_message = Container(
        name="hello-world-with-time",
        image="busybox",
        command=["echo"],
        args=["🕓 hello world. Scheduled on: {{workflow.scheduledTime}}"],
    )

w.create()

Expected behavior
A clear and concise description of what you expected to happen:

• The response should be able to decode as an AW 3.5 object

Environment

• Hera Version: 5.19.0
• Python Version: 3.X.X
• Argo Version: 3.5.X

Additional context
Add any other context about the problem here.

Seen in https://cloud-native.slack.com/archives/C03NRMD9KPY/p1739305360122419