Fix K8s executor pod_override stringified without cncf provider

[ephraimbuddy]

Airflow 3 deployments that run the Kubernetes executor with the kubernetes
package installed but without apache-airflow-providers-cncf-kubernetes
had their executor_config pod_override V1Pod silently converted to a
string during Dag serialization. The stringified pod was persisted to
serialized_dag._data and task_instance.executor_config, so affected tasks
stayed in queued and never ran. (Airflow 2 handled this case via an in-core
PodGenerator fallback that was removed ahead of the 3.0 release.)

Root cause

_has_kubernetes() in serialized_objects.py hardcoded an import of the cncf
provider's PodGenerator for V1Pod ser/deser. When that provider isn't
installed the import fails, _has_kubernetes() caches False, the V1Pod
serialization branch short-circuits, and default_serialization() falls back
to str(var).

Fix

Serialize/deserialize V1Pod objects directly through
kubernetes.client.ApiClient — which is all PodGenerator.serialize_pod and
PodGenerator.deserialize_model_dict wrap anyway — so airflow-core no longer
depends on any specific Kubernetes-executor provider. V1Pod ser/deser now
works with the cncf provider, an alternative executor package, or just the bare
kubernetes package installed. The backcompat unpickle path in
ensure_pod_is_valid_after_unpickling is decoupled the same way.

Tests

Added a regression test that makes apache-airflow-providers-cncf-kubernetes
unimportable and asserts V1Pod still round-trips through
BaseSerialization. The test blocks the ...pod_generator submodule directly
(not just the parent package) so it can't be fooled by import ordering when the
module is already cached by a sibling test. Existing K8s serialization tests
(test_serialize_v1pod_attempts_import_before_serializing, the
executor_config_pod fixtures, TestExecutorConfigType) continue to pass —
serialization output is byte-identical to the previous cncf-based path.

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• Yes — Claude Code (Opus 4.8)

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