Add NICo node maintenance validation

[osu]

Summary

• replace the BFX01-02 NICo gap stub with a real Machine maintenance-mode cycle
• require an explicit dedicated Machine ID and a separate mutation opt-in; default runs remain read-only structured skips
• reject machines that are not Ready, belong to another Site, or are allocated
• request maintenance through NICo, verify the Machine reaches Maintenance, and always disable maintenance mode in finally
• verify the fixture returns exactly to its initial Ready state before reporting success
• strengthen ReturnNodeMaintenanceCheck so fabricated accepted: true output cannot pass without request, state, and restoration evidence

Safety

This is staging-only and mutating. It runs only when both are set:

• NICO_BREAKFIX_MACHINE_ID=<dedicated-fixture-id>
• NICO_BREAKFIX_ALLOW_MUTATION=1

No Machine is selected automatically. The orchestration timeout allows the cleanup path to finish.

Closes #207.

Validation

• uv run pytest -q isvctl/tests/providers/nico/test_nico_provider.py isvtest/tests/test_breakfix.py (199 passed)
• all my-isv demo suites passed
• NICo configuration and suite wiring validation passed
• applicable pre-commit hooks passed
• default live invocation against dev3 returned the intended read-only fixture-required runtime skip

No live maintenance mutation was run because the six visible dev3 Machines are not identified as dedicated/reserved validation fixtures.

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