test: add direct unit coverage for closeDbPair/closeDbPairDeferred/closeDbDeferred

[carlos-alm]

Summary

• closeDbPair, closeDbPairDeferred, and closeDbDeferred had zero direct test coverage despite being core resource-lifecycle primitives in a fanIn-55 file — exactly the kind of gap an earlier phase's real resource-leak bug slipped through.
• Added 10 tests to tests/unit/db.test.ts (existing home for this file's connection tests): close ordering (native before better-sqlite3, survives native-close failure), deferred-close timing (native immediate, better-sqlite3 deferred to next tick), and advisory-lock release/flush semantics (immediate lock release, flushDeferredClose() closes synchronously with no double-close).
• No production code changes — everything needed was already exported.

Closes #1751

Test plan

• npm test — full suite green (3500 passed, 0 failed), all 10 new tests confirmed running and passing (not skipped)
• npm run lint / npm run typecheck — clean
• Non-vacuousness confirmed via 3 temporary mutation checks (swapped close order, removed double-close guard, moved lock release inside the deferred callback) — each correctly caught by the corresponding test, then reverted

Stacked on #1861 (base branch fix/issue-1750-finddbpath-swallowed-errors) — only the test diff is this issue's change.

[greptile-apps]

Greptile Summary

This PR adds 10 unit tests covering three previously untested resource-lifecycle helpers — closeDbPair, closeDbPairDeferred, and closeDbDeferred — all in the existing tests/unit/db.test.ts file. No production code is changed.

• closeDbPair (3 tests): verifies native-before-better-sqlite3 close ordering, error resilience when the native handle throws, and graceful handling of a missing native handle.
• closeDbPairDeferred (3 tests): confirms the native close is synchronous while the better-sqlite3 WAL checkpoint is deferred to the next tick via setImmediate, including the throw-resilient path.
• closeDbDeferred (4 tests): covers advisory-lock release timing (synchronous), deferred handle close (next tick), flushDeferredClose() short-circuit with no double-close, and the no-lock-path path.

Confidence Score: 5/5

Pure test addition with no production code changes; all new tests correctly model the source functions they exercise.

Every new test was traced against the corresponding implementation in connection.ts — close ordering, try/catch resilience, setImmediate deferral, the double-close guard (indexOf check), and advisory-lock release timing all match the production code exactly. The afterEach cleanup pattern correctly drains the module-level _deferredDbs singleton between tests. No production code was touched.

No files require special attention.

Important Files Changed

Filename	Overview
tests/unit/db.test.ts	Adds 10 focused unit tests for closeDbPair/closeDbPairDeferred/closeDbDeferred using lightweight mock handles; correct cleanup via afterEach, accurate tick() implementation, and proper double-close guard verified.

_{Reviews (1): Last reviewed commit: "test: add direct unit coverage for close..." | Re-trigger Greptile}

[carlos-alm]

Recreated as #1910 — this PR was inadvertently closed when its stacked base branch was deleted after #1903 merged. Continuing there.