fix(tests): restore run_ssh stub attribute for installer environment tests

[mrbobbytables]

What

Fixes #762 — Unit Tests failing on main with:

AttributeError: <module 'tests.shared.ssh_steps'> has no attribute 'run_ssh'

raised from tests/unit/test_installer_environment.py when it does
monkeypatch.setattr(ssh_steps, "run_ssh", fake_run_ssh).

Root cause

Three unit-test import helpers replace sys.modules["tests.shared.ssh_steps"]
with a bare stub module and never restore the original afterwards:

• tests/unit/test_dx_steps.py
• tests/unit/test_software_steps.py
• tests/unit/test_flatpak_permissions_steps.py

None of these stubs set a run_ssh attribute. Since the CI job runs
pytest -n auto (pytest-xdist), whichever worker executes one of these three
tests first permanently corrupts that worker's sys.modules entry for
tests.shared.ssh_steps for the remainder of the session — there's no
teardown restoring the real module.

tests/unit/test_installer_environment.py later does
import tests.shared.ssh_steps as ssh_steps and expects to be able to patch
ssh_steps.run_ssh. When it runs in a worker whose ssh_steps module was
already replaced by one of the bare stubs above, monkeypatch.setattr raises
AttributeError because the stub has no run_ssh attribute at all.

This reproduces locally with coverage run -m pytest -n auto tests/unit/
(consistent 4 failures), but passes when the installer environment test file
is run in isolation — a classic cross-test pollution bug that only shows up
under parallel/xdist ordering.

Fix

Give each of the three offending stubs a run_ssh MagicMock, matching the
pattern already used by every other suite's stub in this file
(hardware, flatcar, nvidia, lifecycle, offline_boot,
update_failure, common_steps), so the stubbed module always exposes the
attribute that other tests expect to find or patch.

Testing

• python3 -m pytest -n auto tests/unit/ -q — 1527 passed (run 3x to rule out
  xdist ordering flakiness); previously reproduced 4 failures in
  test_installer_environment.py under the same command before this fix.
• ruff check on the three changed files shows the same pre-existing findings
  (import sorting / SIM117 / stale noqa) as on main before this change —
  none introduced by this patch.

Note: I based this branch on main since v4 does not exist in
projectbluefin/testsuite (verified via git ls-remote --heads upstream);
main is the repository's only long-lived/default branch.

— hive: backend=copilot model=claude-sonnet-5

🐝 Hive Agent: contributor | SHA: e67f900b

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[kubestellar-hive]

Approved by @castrojo for Hive auto-merge on green CI.