fix(ACI): Parse and persist owner field

[ceorourke]

Follow up to #110785 and #110381 to write and update the owner on a workflow if that data is passed to the legacy endpoint and we have the feature flag flipped so that we single write.

[linear-code]

ISWF-2237 Make sure owner fields are created/updated for workflows

[github-actions]

Backend Test Failures

Failures on d0e5fbb in this run:

tests/sentry/api/endpoints/test_project_rule_details.py::UpdateProjectRuleTest::test_update_owner_type — log

tests/sentry/api/endpoints/test_project_rule_details.py:752: in test_update_owner_type
    workflow = Workflow.objects.get(id=workflow_response.data["id"])
.venv/lib/python3.13/site-packages/django/db/models/manager.py:87: in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
.venv/lib/python3.13/site-packages/django/db/models/query.py:635: in get
    raise self.model.DoesNotExist(
E   sentry.workflow_engine.models.workflow.Workflow.DoesNotExist: Workflow matching query does not exist.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Owner clearing fails when explicitly set to None

Medium Severity

The if owner: check in format_request_data treats an explicitly-passed None value the same as a missing key. When a user sends "owner": null to clear an existing owner, data.get("owner") returns None, the falsy check skips the block, and workflow_payload never receives an owner key. The WorkflowValidator.update method then sees "owner" not in validated_data and preserves the old owner. The downstream update_owner function correctly handles None by clearing both owner_user_id and owner_team_id, but it's never reached. The existing test_no_owner test doesn't catch this because the workflow starts without an owner. A check like if "owner" in data with separate handling for None vs. truthy values is needed.