Fix `@task.kubernetes_cmd` TaskGroup.expand mappings by templating TaskFlow args

[akkik04]

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Closes #56459

Problem

@task.kubernetes_cmd breaks when used with TaskFlow-style mapping inside a TaskGroup. The root cause is that we didn't mark TaskFlow arguments (op_args / op_kwargs) as template_fields, thus the validation of the return value of the Python callable occurs before those fields are resolved by the TaskFlow/mapping machinery.

As a result, when group_task.expand(...) is used, the name parameter that reaches the task (echo_cmd is the task being used in the repro linked within the issue) is still a MappedArgument object, and the type check inside _generate_cmds sees a non-str element and raises the TypeError: Expected echo_cmd to return a list of strings, but got ['echo', MappedArgument(...)].

Solution

I aligned @task.kubernetes_cmd with @task.kubernetes with regards to how templated fields are handled, so TaskFlow mappings behave consistently. Concretely, I extended template_fields to include op_args and op_kwargs alongside the existing Kubernetes-specific fields inherited from KubernetesPodOperator.template_fields. This ensures that TaskFlow arguments—including mapping metadata such as MappedArgument coming from TaskGroup.expand(...) will be run through the normal TaskFlow resolution scheme and reach the user function as plain Python values instead of unresolved mapping objects.

Finally, I add a focused unit test, test_kubernetes_cmd_template_fields_include_taskflow_args, which creates a simple @task.kubernetes_cmd TaskFlow function inside self.dag_maker, instantiates it, and asserts that the underlying operator’s template_fields include at least "op_args" and "op_kwargs". This guards against future refactors accidentally dropping TaskFlow arguments from template_fields and reintroducing the original MappedArgument type error for TaskGroup.expand(...) with @task.kubernetes_cmd.

Open to discussion!

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