`error_type` becomes a versioned vocabulary: a failure the classifier cannot place is stored as `unknown` instead of NULL, and any stored value outside the vocabulary reads back as `unclassified` in the breakdown.
Maintainer rework before merge (kept the idea, no default behavior change):
- `PROVIDER_ERROR_TYPES` is `as const`, so `ErrorTypeContract` is a real union and the classifier functions return the narrowed type.
- The constants moved above the JSDoc that documents `getErrorTypeBreakdown`; the vocabulary SQL is built on first use so an import cycle cannot read it before it exists.
- Legacy NULL rows keep landing in the `pre_migration`/`unclassified` bucket without vanishing or double counting, and the log export / BigQuery row pass both NULL and `"unknown"` through unchanged — both covered by tests. Duplicate assertions removed; every seeded row is cleaned up in `finally`.
Validated first on the combined board of all 38 PRs of this batch (10 merged as-is, 28 after the maintainer rework) on top of release/v3.8.51 c0f92ec: typecheck:core, check:open-sse-typecheck and check:dashboard-typecheck clean; ESLint clean on every changed file; file-size (rebaselined for the combined growth), complexity, cognitive-complexity, changelog-integrity, docs-counts, docs-sync, migration-numbering and i18n new-key gates green; 735 focused node:test cases with the only batch-caused failure (a flag-count assertion) fixed. Then re-validated alone on the fresh release tip right before this merge: ESLint on the changed files, typecheck:core, check:open-sse-typecheck, the file-size/complexity/changelog gates and this PR's own tests.
Thanks @maxmad64bis!
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