Fix #1267: [torchx/schedulers] Add support for kueue_scheduler

[JiwaniZakir]

Closes #1267

Adds KueueScheduler as a namespace-package plugin under torchx_plugins/schedulers/kueue.py, implementing the plugin architecture described in the issue. KueueScheduler subclasses KubernetesScheduler and overrides _submit_dryrun to inject the kueue.x-k8s.io/queue-name label into the job resource metadata before submission. The queue name is read from cfg.get("queue"), defaulting to "default" if unset. The factory function kueue_scheduler is registered via @register.scheduler(name="kueue"), making it auto-discoverable at runtime when the wheel containing torchx_plugins/ is installed.

Test plan:
Two unit tests added to torchx/schedulers/test/kubernetes_scheduler_test.py under KueueSchedulerTest:

• test_factory_returns_kueue_scheduler: calls kueue_scheduler(session_name="test") and asserts the returned object is a KueueScheduler instance with session_name == "test".
• test_queue_label_added_to_metadata: patches KubernetesScheduler._submit_dryrun with a MagicMock, invokes KueueScheduler(session_name="test")._submit_dryrun with Opts(queue="my-kueue-queue"), and asserts that resource["metadata"]["labels"]["kueue.x-k8s.io/queue-name"] == "my-kueue-queue" and that the original dryrun_info object is returned unmodified.

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This PR was created with AI assistance (Claude). The changes were reviewed by quality gates and a critic model before submission.