test: add unit tests for KubeflowExecutor and KubeflowScheduler

63 tests covering:
- Executor: defaults, kubeconfig fallback, nnodes, nproc_per_node resolution,
  assign, manifest generation for PyTorchJob and TrainJob (structure, resources,
  volumes, env_vars, env_list, labels, image_pull_secrets, tolerations, affinity,
  pod_spec_overrides, spec_kwargs, container_kwargs), launch (success, wait,
  timeout, conflict), status (all states + API errors), cancel (plain, 404,
  wait=True, wait timeout), fetch_logs (no-follow, follow, TrainJob label selector)
- Scheduler: create, dryrun, schedule, describe (all states + UNKNOWN→PENDING
  regression), cancel, log_iter (list + str), persistence (new file, merge,
  missing file), state map

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: oliver könig <okoenig@nvidia.com>

Author: ko3n1g