CloudKitSyncEngine.currentAccountId returns nil while peer methods throw accountUnavailable

[datlechin]

Follow-up to #1028.

CloudKitSyncEngine has six methods that touch the optional container/database. Five of them (checkAccountStatus, ensureZoneExists, pushBatch, performPull, plus push's wrapping callsite) throw SyncError.accountUnavailable when the entitlement is absent. The sixth, currentAccountId, returns nil instead:

// TablePro/Core/Sync/CloudKitSyncEngine.swift
func currentAccountId() async throws -> String? {
    guard let container else { return nil }
    return try await container.userRecordID().recordName
}

The single caller (`SyncCoordinator.currentAccountId` → `checkAccountStatus` at `TablePro/Core/Sync/SyncCoordinator.swift:632`) wraps it in `try? await`, so both shapes happen to work today. But the inconsistency makes the actor's contract harder to reason about: "missing entitlement" surfaces as two different things depending on which method you call.

Fix

Make currentAccountId throw, matching its peers:

func currentAccountId() async throws -> String? {
    guard let container else { throw SyncError.accountUnavailable }
    return try await container.userRecordID().recordName
}

Flip the matching test in TableProTests/Core/Sync/CloudKitSyncEngineTests.swift from currentAccountIdReturnsNil to expect a throw, mirroring the other five test cases.

The existing try? at the call site stays correct (it already swallows throws).

~5 lines of source + the test rename.