[cncf-kubernetes] Defer kubernetes_asyncio import in callbacks to TYPE_CHECKING

[dor-bernstein]

What

Move the kubernetes.client and kubernetes_asyncio.client imports — and the client_type type alias — in providers/cncf/kubernetes/.../callbacks.py under TYPE_CHECKING.

Why

Importing airflow.providers.cncf.kubernetes.operators.pod (the base KubernetesPodOperator) transitively imports callbacks.py. That module eagerly ran import kubernetes_asyncio.client (and import kubernetes.client) at module scope, so every DAG that builds a KubernetesPodOperator at parse time paid the cost of loading the async client — which pulls in aiohttp + ~625 generated model modules (~0.2s) and is only actually needed at runtime in the triggerer for deferrable pods.

The two client imports were used only for:

1. the module-level client_type: TypeAlias = k8s.CoreV1Api | async_k8s.CoreV1Api, and
2. parameter annotations on the callback methods.

Since the module uses from __future__ import annotations (PEP 563), all those annotations are strings and are never evaluated at runtime — so the imports and the alias only need to exist for static type checkers.

Change

Both client imports and the client_type alias are moved into the existing if TYPE_CHECKING: block. No public behavior changes: ExecutionMode and KubernetesPodOperatorCallback still import normally; client_type has no runtime importers (only ExecutionMode / KubernetesPodOperatorCallback are imported from this module, by operators/pod.py and utils/pod_manager.py).

Verification

Loading the module standalone before/after:

	kubernetes_asyncio in sys.modules after import
before	True
after	False (and kubernetes: False)

ExecutionMode.ASYNC and KubernetesPodOperatorCallback still resolve correctly after the change.

Note: opened against the clutch fork. Provider CI / breeze static checks (ruff, mypy) should run before merge.