Add unit tests for ironic.py pure helpers

[berendt]

Closes #2226.

Adds tests/unit/tasks/conductor/test_ironic_helpers.py covering the pure
helpers and the _prepare_node_attributes builder in
osism/tasks/conductor/ironic.py — everything testable without the full
sync orchestration (the sync entry points stay in the companion sub-issue).

Change set

A single new test module (no production code touched), organized by helper:

• _derive_as_from_hostname_yrzn — storage vs. non-storage type digit,
  < 5 parts → None, single-digit padding, already-two-digit values left
  alone.
• _get_metalbox_primary_ip4_fallback — invalid YAML, non-list setting,
  non-dict element skipped, tag stripped + role="metalbox" added (asserted
  via captured filter kwargs), first/second metalbox IP selection, and the
  two no-IP / no-metalbox warning paths.
• _get_metalbox_primary_ip4 — no OOB IP (fallback not called), subnet
  match, no-match → fallback, matching subnet but no primary_ip4 → early
  None (no fallback), match on the second interface, and IPv4/IPv6 mixing.
• _render_templates — flat/nested dict, nested list, list of dicts,
  non-template and non-string values untouched, multiple vars, in-place
  mutation returning None.
• _prepare_node_attributes — base/config-context/custom-field merging
  and order, driver pruning, template variables (board creds, OOB address,
  ironic_osism_* propagation), osism-ipa-type=yrzn001 kernel enrichment
  (frr params, metalbox, AS fallback, unknown type), skip_kernel_params,
  extra_kernel_params, driver_info persistence, extra serialization, and
  the returned tuple.
• _prettify_for_display — JSON parsing in extra, non-JSON left
  untouched, deep copy with/without extra.

Mocking notes

• Module-level collaborators (deep_decrypt, deep_merge, get_vault,
  get_device_oob_ip, SUPPORTED_IPA_TYPES, _derive_as_from_hostname_yrzn,
  _get_metalbox_primary_ip4) are patched at osism.tasks.conductor.ironic.*.
• The NetBox client is patched via osism.utils.nb (with create=True), which
  is what the helpers' local from osism import utils resolves to — matching
  the existing test_netbox.py and sonic test fixtures. ironic.py has no
  module-level utils attribute (it imports it as osism_utils), so the
  literal osism.tasks.conductor.ironic.utils.nb path from the issue is not a
  valid patch target; osism.utils.nb is the established equivalent.
• deep_decrypt / deep_merge are stubbed as no-ops; their real behavior is
  covered by the conductor.utils tests, so the merge cases assert call
  identity/order rather than merged output.

Deviation from the issue

The issue lists _derive_as_from_hostname_yrzn("comp-nw-22-3-7-1") →
"4200143307". The implementation pads rack/server and produces
"4200140703" (matches the function's own docstring example
stor-nw-22-60-59-6 → 4200155960). The test encodes the verified value
"4200140703"; the same case (non-storage type 4, single-digit padding) is
still covered.

Verification

• black --check, flake8 — pass on the new file.
• pytest / coverage — left to the python-osism-unit-tests Zuul job.

AI-assisted: Claude Code

[sourcery-ai]

Hey - I've left some high level feedback:

• The local _YRZN_MAPPING test fixture duplicates the SUPPORTED_IPA_TYPES structure and can silently diverge from the real mapping over time; consider deriving it from SUPPORTED_IPA_TYPES at runtime (e.g. SUPPORTED_IPA_TYPES['yrzn001'].copy() or similar) and only overriding what needs to be controlled for the tests.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The local `_YRZN_MAPPING` test fixture duplicates the `SUPPORTED_IPA_TYPES` structure and can silently diverge from the real mapping over time; consider deriving it from `SUPPORTED_IPA_TYPES` at runtime (e.g. `SUPPORTED_IPA_TYPES['yrzn001'].copy()` or similar) and only overriding what needs to be controlled for the tests.

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