fix: kubernetes exectutor health check uses out-of-scope command

[Danelegend]

Currently the health check of the kubernetes executor uses read_namespace. This uses additional rbac permissions. We want to limit the amount of permissions. This change uses the create verb to check if we have access to that resource (which we need to create sub-pods).

*   Trying 127.0.0.1:8000...
* Connected to 127.0.0.1 (127.0.0.1) port 8000
> GET /health HTTP/1.1
> Host: 127.0.0.1:8000
> User-Agent: curl/8.7.1
> Accept: */*
> 
* Request completely sent off
< HTTP/1.1 200 OK
< date: Fri, 27 Feb 2026 18:49:40 GMT
< server: uvicorn
< content-length: 79
< content-type: application/json
< 
* Connection #0 to host 127.0.0.1 left intact
{"status":"error","message":"Kubernetes API error (namespace=onyx): Forbidden"}%