S4 — Retire the _validate replica: test the gate that exists, where it lives (C-03) #185

Part of epic #181. Closes register C-03 (Tier 3).

Background

tests/test_validation.py opens with this:

"Note: views-pipeline-core may not be installed in test environments, so we replicate the validation logic here rather than importing the manager class. The logic tested matches unfao.py:_validate() exactly."

That claim is now false, and has been since #149.

The _validate that existed when the file was written null-gated the nine GAUL metadata columns. The _validate that exists today does not — its own docstring says so:

def _validate(self) -> None:
    """Assert the read produced what the save needs, then check coverage.

    Neither payload is null-gated here. The historical artifact's metadata
    null-gate fires at build time (``historical.assert_metadata_complete``);
    ...
    """

It now asserts that _read resolved (_historical_frame is not None, _forecast_resolution is not None) and calls _check_coverage. The null gate moved to contract/historical.assert_metadata_complete during the frame-native migration.

So the test file:

1. tests a gate that no longer lives where it says,
2. asserts fidelity to a method it no longer resembles,
3. hardcodes REQUIRED_METADATA_COLS as a 9-element literal instead of importing gaul_schema.METADATA_COLS — the exact single-source-of-truth defect C-70 was resolved to prevent.

Point 3 already has a witness in the suite: tests/test_falsification_campaign_4_3.py:32 notes "The validation tests use a hardcoded REQUIRED_METADATA_COLS list that…". Someone saw it and moved on.

C-03's registered residual is exactly these two gaps: "the standalone _validate() replica and the missing enrich→validate end-to-end test." The first has since decayed from a duplication risk into an active falsehood.

Work

1. Delete the replica's false claim and the duplicated column list. Import METADATA_COLS from views_postprocessing.contract.gaul_schema. The list stops being a copy.
2. Re-point the null-gate tests at the module that owns the gate. The behaviour C-01 fixed (null validation, and the failure messages the CIC promises) is real and worth keeping — it just lives in contract/historical.assert_metadata_complete now. Test it there, directly, with no replica: that module is dependency-light and imports fine without pipeline-core, so the original justification for replicating no longer applies either.
3. Correct the module docstring to say what the file now tests and where the gate lives.
4. Close C-03's second residual — the missing enrich→validate end-to-end path — or state in the register why it stays open. test_historical_parity.py and test_historical_builder.py may already cover it end to end via the frame-native path; check before writing a new test. Do not add a redundant test to close an entry.

Acceptance criteria

• No hardcoded copy of the nine metadata column names anywhere in tests/.
• No test file claims to replicate a manager method.
• The null-gate behaviour is asserted against historical.assert_metadata_complete, the module that performs it.
• _validate's current contract (both resolution assertions + the coverage call) has at least one test, or the register states why it does not.
• C-03 is closed with the command that proves it, or its remaining scope is narrowed in writing.

Testing

ruff check .
pytest -q tests/test_validation.py tests/test_historical_builder.py tests/test_historical_parity.py
pytest -q

Sanity check that the replica is really gone:

grep -rn "REQUIRED_METADATA_COLS\|matches unfao.py" tests/

Dependencies

None.

Files

• tests/test_validation.py — the replica and the false claim
• views_postprocessing/unfao/managers/unfao.py:228-262 — _validate / _check_coverage as they actually are
• views_postprocessing/contract/historical.py — assert_metadata_complete, where the gate lives
• views_postprocessing/contract/gaul_schema.py:48 — METADATA_COLS, the declared contract (C-70)
• tests/test_falsification_campaign_4_3.py:32 — the earlier witness
• docs/CICs/UNFAOPostProcessorManager.md — check it does not repeat the stale claim