Stabilize empty plugin update scheduler regression

[ymichael]

What was wrong

The empty-plugin scheduler regression inherited the file's heavyweight Git-plugin integration fixture. Before it could assert the scheduled delay, the test installed and loaded a Git plugin, then called the public removal path solely to manufacture an empty database. That removal performs lifecycle disposal, managed-file cleanup, and CLI-skill synchronization unrelated to periodic scheduling. The passing main run already spent about 3.3 seconds in this test; locally it reached 4.790 seconds, and under ordinary contention the removal did not finish before Vitest's 5-second default timeout. The scheduler assertion itself is synchronous and deterministic.

What changed

Move the empty-set scheduling regression into a small fixture that owns an in-memory SQLite database with no installed plugins and injects the scheduler callback directly. The assertion remains exact: starting periodic checks with no eligible rows must schedule one six-hour delay. The fixture explicitly stops its service and closes its database.

This is test-only. There is no server/daemon wire change, CLI or configuration surface, or documentation change.

How you verified

• Before the change, the focused test took 4.790 seconds locally; an instrumented rerun reproduced the Test timed out in 5000ms failure before service.remove("updater") completed.
• After the change, the focused regression passed in 61ms. With 24 CPU workers started alongside Vitest, it passed in 74ms while retaining the exact six-hour assertion.
• pnpm exec turbo run test --filter=@bb/server --force -- test/services/plugins/plugin-update.test.ts — 27 tests passed.
• pnpm exec turbo run test --filter=@bb/server --force — 210 test files passed, 1 skipped; 2,002 tests passed.
• pnpm exec turbo run typecheck --filter=@bb/server — passed.
• pnpm exec turbo run build --filter=@bb/server — passed.
• git diff --check — passed.

No matching GitHub issue was found, so this PR does not include a Fixes #N reference.

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