test(server): pin migration invariants + embedding-model literal drift guard

[buremba]

Summary

Follow-up to the 2-week stability audit. Adds two additive CI guards (no prod-code behavior change) so recently-changed, load-bearing invariants can't silently regress in a future migration — directly serving the "reliable and consistent tests" goal.

• migration-invariants.test.ts (integration, runs the real baseline→latest chain the harness already applies):
  • asserts the per-user pending oauth_account unique index from fix(server,db): per-user pending oauth_account uniqueness + clean conflict errors #1121 exists and the old org-wide auth_profiles_pending_unique is gone
  • functional contract: a user's second parallel pending OAuth flow collides (PendingAuthConflictError) while a distinct user's is allowed — the actual fix(server,db): per-user pending oauth_account uniqueness + clean conflict errors #1121 guarantee, not just the DDL
  • asserts event_embeddings.embedding_model stamp column exists (fix(embeddings): version-stamp embeddings and batch the sync embed path #1069/fix(embeddings): stamp legacy embedding_model + stop liveness probe killing the embeddings service #1080)
• embedding-model-literal.test.ts (unit): the legacy-stamp backfill migration hard-codes the model literal (SQL can't import TS). This fails if it ever drifts from DEFAULT_EMBEDDING_MODEL, which would silently re-open the full-corpus recall regression. Exports DEFAULT_EMBEDDING_MODEL for the assertion.

Audit context

The #1121 "dropped pending-uniqueness for browser_session" concern turned out to be a false positive on closer reading: browser_session rows are created with connector_key = NULL (manage_auth_profiles.ts:696), and the old dropped index was a standard unique index — Postgres treats NULLs as distinct, so it never enforced uniqueness for that kind. oauth_account (the only reachably-pending kind with non-null keys) is correctly covered. So no code guard was added; the real contract is pinned by the functional test instead.

Test plan

• vitest run both files green (5 integration + 1 unit) against embedded Postgres
• red→green proof: drifting DEFAULT_EMBEDDING_MODEL makes the literal guard fail; reverting restores green
• tsc --noEmit clean (server); pre-commit biome + typecheck pass

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 36cc4706-2e70-4fa9-8590-e7646a7745a4

📥 Commits

Reviewing files that changed from the base of the PR and between 9c5a3ce and 860eb25.

📒 Files selected for processing (3)

• packages/server/src/__tests__/integration/db/migration-invariants.test.ts
• packages/server/src/utils/__tests__/embedding-model-literal.test.ts
• packages/server/src/utils/embeddings.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/stability-test-hardening

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