fix(db): return undefined for empty bun:sqlite .get() results

[Ercaner1988]

Summary

• bun:sqlite's Statement.get() returns null when no row matches. better-sqlite3, node:sqlite and sql.js return undefined, and the rest of the code is written against that (get(...) as Row | undefined; isExclusiveConnectionActivelyLeased compares with !== undefined).
• Under Bun the lease guard therefore reports every connection as exclusively leased. Observed on a Bun-run server (a v3.8.50 install; the same code is on release/v3.8.51): the dashboard connection test returned LEASE_ACTIVE (skipped, 0 ms) for every connection tried (antigravity and mistral), usage refresh was deferred with 409, and the model-sync scheduler logged No connections with autoSync enabled, while exclusive_connection_leases was empty and no API key had the lease:exclusive scope.
• The Bun adapter now maps the no-row null to undefined, so all drivers share one contract. The change is confined to the best-effort Bun adapter; the Node drivers are untouched, and it does not change the runtime support policy (Node remains the supported runtime).
• Checked on Bun 1.4.0 and 1.4.2: both return null for a .get() with no matching row.

Related Issues

• Related to fix(bun): use native bun:sqlite during startup to avoid N-API crash #11468 and feat: support Bun bundled SQLite runtime #7878 (Bun SQLite adapter work)

Validation

Choose the change type and focused loop from the
Contribution Golden Path. The full unit suite,
Vitest, the 60% coverage gate, and the production build all run in CI on this PR (#8329):

• Change type: DB (Bun adapter)
• Focused tests and category gates from the golden path: check-db-rules passes; the test:bun:db file set (bunSqliteAdapter, driverFactory, cliSqlite) passes 24/24 on Bun 1.4.2. The two new tests failed before the change (actual: null, expected: undefined) and pass after it.
• npm run lint: ESLint (repo config and suppressions) and Prettier were run on the two changed files and are clean. The full-repo lint is left to CI.
• Reconciled with the current active release base: the branch was cut from the release/v3.8.51 tip (7a921299c, 2026-09-19).
• Production-code changes include a new or updated automated test in this PR
• SonarQube is temporarily opt-in while the private project has no quota; it is not a PR gate.

Tests Added Or Updated

• tests/unit/db-adapters/bunSqliteAdapter.test.ts: two new tests. One uses a fake driver whose get() returns null, so it runs under any runtime, including the Node shards. The other runs against the real bun:sqlite and is skipped outside Bun (it is picked up by test:bun:db).

Coverage Notes

• src/lib/db/adapters/bunSqliteAdapter.ts changes by one expression. The fake-driver test covers it on every runtime; the real-driver test covers it under Bun.
• Not run locally: the Node unit shards, typecheck:core, the full-repo lint and the production build. CI runs them on this PR.

Reviewer Notes

• ⚠️ base-red inherited: 🔴 Release branch not green: release/v3.8.51 #13866
• The fix is at the adapter on purpose: the !== undefined checks and the | undefined typings elsewhere assume the shared contract, so normalizing once here covers every caller instead of patching call sites one by one.
• No migration or feature flag.

[diegosouzapw]

Thanks @Ercaner1988 — merging via the release merge-train. Validated in local merge-train (mt-train10c) on the devbox @ train tip 4d841aa1c740bbaa03868dc0a403c62099a99a42 with the 72 sibling PRs of this batch: typecheck:core, file-size, complexity, cognitive-complexity, changelog-integrity green; changed-area node:test 831/831 (0 failing) and vitest 480/482 — the two reds are autoCombo/provider-family-combos.test.ts timing out at 20s, which reproduces on the PURE release tip under the full vitest suite (and is already tracked by the Release-Green issue #13866), so it is inherited, not this batch's. Merged --admin per merge-gates §3/§4/§7.