fix: UK VAT service 404 (not found) misreported as service outage

[akinsteph]

AI-generated PR. This pull request was implemented by an AI agent (Claude Sonnet 4.6) following a groomed plan for issue #14. Please review carefully before merging.

Summary

Fixes #14.

HMRCRegistry.check_vat_number treated an HTTP 404 from the HMRC VAT-check API exactly like a generic outage (timeout, 500, malformed response, wrong content type) — logging a "Response is nondeterministic..." line and returning is_valid=False with no way for a caller to distinguish "VAT number not found" from "the service is down."

A 404 from this API is a deterministic answer, not an outage: the VAT number does not exist in HMRC's registry. This PR adds a dedicated branch that catches status_code == 404 before the existing generic status/content-type check, logs a plain "VAT number not found (404 NOT_FOUND)" line instead, and returns is_valid=False with no outage/nondeterministic signal.

All other error paths (timeouts, 500s, non-JSON content type, other non-200 statuses) are untouched, so downstream outage detection (e.g. wp-media/monies, which greps log_lines for "Response is nondeterministic") keeps working exactly as before.

Changes

• pyvat/registries.py: added a response.status_code == 404 branch in HMRCRegistry.check_vat_number, placed immediately before the existing if response.status_code != 200 or ... outage check. No signature or public API changes.
• tests/test_hmrc_registry.py (new): regression tests for the HMRC registry using unittest.mock.patch on requests.get (no live HMRC calls), covering:
  • 404 → is_valid=False, no nondeterministic/exception/ServerError signal in log_lines
  • 500 → is_valid=False, nondeterministic present in log_lines (outage path still works)
  • 200 with a valid target payload → is_valid=True, business_name/business_address populated
  • 200 with a non-JSON content type → still flagged nondeterministic
  • Timeout → still flagged as a timed-out request

Acceptance criteria

• A 404 response results in is_valid=False without any exception, ServerError, or nondeterministic/service-unavailable signal in result.log_lines.
• A genuine outage (timeout, 500, malformed/non-JSON response, non-404 non-200 status) still results in a nondeterministic log line.
• Regression tests cover 404, 500, and 200 cases.

Test plan

• python -m pytest tests/ -v — 21 passed, 2 skipped (pre-existing, network-dependent live-VAT-check tests unrelated to this change).
• Manual smoke test through the public pyvat.check_vat_number() entry point with a mocked 404 response, confirming is_valid=False and no outage signal in log_lines.
• flake8 run on the changed file — no new violations introduced (pre-existing violations on unrelated lines untouched).

Notes

• No public API surface changed (no new endpoints, config keys, or signature changes), so no documentation updates were needed.
• This repo had no existing mocking convention for HTTP calls in tests; unittest.mock.patch on requests.get was used for the new test file, seeding access_token on the registry instance in setUp so the _authenticate() retry path (which itself makes an HTTP call) doesn't need to be mocked for these tests.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

[nicomollet]

@akinsteph Looks good we would be more confortable with a test on Monies staging.

Create an issue for Monies (update Pyvat) and PR pointing to this Pyvat commit and test with a UK/GB VAT number:

---

Monies staging to test

curl --location 'https://monies-staging.wp-media.me/api/vat/vat-rate/?country_code=GB&vat_number=GB674107243' \
--header 'Content: application/json' \
--header 'Authorization: token 27129e1267ef2e0f2b12ecbd14e64eb1854ad446' \
--header 'Accept: application/json' \
--data ''

Current behavior on Monies production:

curl --location 'https://monies.wp-media.me/api/vat/vat-rate/?country_code=GB&vat_number=GB674107243' \
--header 'Content: application/json' \
--header 'Authorization: token xxx' \
--header 'Accept: application/json' \
--data ''

{
    "country_vat_rate": 20.0,
    "regional_vat_rate": 20.0,
    "user_vat_rate": 20.0,
    "validation_error": "VAT validation remote service is not available"
}