Environment variables for image_name and executor_image_name not taking precedence

[vegetablest]

Description

The environment variables GP_IMAGE_NAME and GP_EXECUTOR_IMAGE_NAME do not take precedence over configuration defaults when launching container-based kernels.

Reproduce

import os
from uuid import uuid4

from jupyter_client.manager import KernelManager

os.environ['JUPYTER_DEFAULT_PROVISIONER_NAME'] = 'kubernetes-provisioner'
os.environ['KERNEL_NAMESPACE'] = 'default'
os.environ['GP_IMAGE_NAME'] = 'my-image'

km = KernelManager(kernel_name="k8s_python")

km.start_kernel(kernel_id=str(uuid4()))

Expected Behavior

Environment variables should take precedence over configuration defaults to allow flexible runtime overrides.

Context

• Operating System and version: Debian GNU/Linux 12
• gateway_provisioners : 0.4.0
• jupyter_client: 8.6.2
• jupyter_server: 2.14.2