test(breakfix): implement BFX01-04 cordon validation

[osu]

Summary

• replace the my-isv BFX01-04 placeholder with a real, reversible Kubernetes cordon workflow
• prove that the selected node becomes unschedulable, an existing probe remains running, and a new node-targeted probe is rejected by the scheduler
• always remove probe pods and uncordon the node, treating cleanup failures as test failures
• support configured kubectl-compatible commands, optional node/namespace/image selection, and pre-existing GPU-node taints without tolerating the cordon itself
• add unit coverage for success, failure cleanup, pre-cordoned nodes, uncordon failure, and taint handling

Why

PR #562 introduces the BFX01-04 validation contract but leaves the provider script as a dummy-success scaffold. This implements the actual Kubernetes behavior required by #209.

This PR is stacked on #562 because the break-fix validation and provider wiring do not exist on main yet.

Impact

Providers using the my-isv Kubernetes configuration now get evidence-backed BFX01-04 results instead of a placeholder. The workflow requires permission to create/delete pods and cordon/uncordon nodes.

Validation

• live run against an isolated k3s v1.35.0 cluster: all three operation fields returned true
• independently verified the node was Ready and schedulable afterward, with no BFX probe pods remaining
• focused tests: 18 passed
• full make test: 1505 passed, 58 passed, 1323 passed with 175 deselected, and 124 passed
• pre-commit checks passed for both changed files

Closes #209

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