fix(core): add timeout to BackgroundProcessManager.close()

[anivar]

We've been hitting a persistent issue where DataStore gets permanently stuck in "Stopping" state after network drops. Once stuck, every save/query/start throws DataStoreStateError and only an app restart recovers. This is a real problem for our offline-first POS app where network is unreliable.

Traced it down to BackgroundProcessManager.close() — it awaits Promise.allSettled(jobs) with no timeout. The subscription processor never implements the onTerminate handler (there's a TODO at subscription.ts:411), so when network is down those jobs just hang forever. close() never returns, DataStore never transitions out of Stopping.

Related: #13035 #10612 #12359

The fix is small — race _closingPromise against a 10s timeout so close() always resolves. One subtlety: close() is async and ends with return this._closingPromise, which in an async function adopts the promise state. So even if you raced past the await, the return would re-block callers on the original never-settling allSettled. Fixed by reassigning _closingPromise to the race result.

After timeout, zombie jobs stay in the Set but self-clean via the existing .then() in registerPromise when they eventually resolve. This matches what the community has been doing as app-layer workarounds (see #10612).

Ideally the subscription/sync processors would implement onTerminate properly, but that's a bigger change across multiple files. This at least unblocks the stop/start cycle.

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