[BE-124] Unit tests for the external-validation module

[mftee]

Overview

backend/src/external-validation/ has 9 files and 1 entity with no tests. It calls third-party validation providers, so its resilience behaviour is what stands between a slow provider and a hung request.

Tasks

• Mock all outbound HTTP — tests must never call a real provider.
• Test the success path for each configured provider and correct normalization of their differing response shapes.
• Test provider failure, timeout, rate-limit (429), and malformed-response handling.
• Test that one provider failing does not prevent others from returning results.
• Test retry and backoff behaviour if implemented, including that retries are bounded.
• Test that provider credentials never appear in logs or error messages.
• Target ≥75% coverage.

Acceptance Criteria

• npm run test -- external-validation passes at ≥75% coverage with no network access.
• A slow or failing provider cannot hang the request.
• Credentials are proven absent from log output.

Notes for Contributors

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