Cross-node sync insert path is network-only, so CI never exercises it

[tipatterson-dev]

Problem

The only test that drives System.insert_self() against a real server is tests/test_node_to_node_sync.py:83, inside a @pytest.mark.network module. CI runs pytest -m "not network", so that path is never exercised there.

That matters more after #42: insert_self() now raises on a rejected POST, and the cross-node sync flow calls it in a helper (_ensure_target_system) whose failure behavior nobody checks automatically. A regression in the raise/no-raise contract would only be caught by whoever remembers to run the network suite locally against a live OSH on :8282 / :8382.

This isn't a defect in shipped code — it's a coverage gap that lets one back in unnoticed.

Reproduce

uv run pytest -m "not network" --cov=oshconnect.resources.system

insert_self()'s server-facing path shows only the mocked-unit coverage added in #42; the sync helper that calls it shows none.

Possible fix

Either is reasonable:

1. Add a mocked (non-network) test for the cross-node sync insert path, so the contract is guarded in CI and the network test stays a genuine live-server smoke test.
2. Stand up an OSH container in CI and run the network suite on a schedule (nightly) rather than per-PR — heavier, but covers the whole -m network set, not just this path.

(1) is the cheap one and covers the specific regression risk from #42.

Related: #42.