Move _subscription_manager_sync_remote_subscriptions() into SubscriptionManagerService and convert the cron queue drain to a QueueWorker plugin

Problem/Motivation

After the Drupal 12 hook conversion (#3603421: Automated Drupal 12 compatibility fixes for subscription_manager 1.0.x-dev), all hook implementations live in the SubscriptionManagerHooks class, but two legacy procedural patterns remain in the code path they share:

1. _subscription_manager_sync_remote_subscriptions() is still a procedural helper in the .module file. It is domain logic (reconcile a user's remote billing contracts into local subscription entities), not glue: keeping it procedural leaves it dependent on \Drupal:: statics, hard to unit test, and stranded in a file that should now contain only #[LegacyHook] shims and form-submit callbacks. Both of its callers (userInsert() and the cron queue drain) are already methods on the hooks class.
2. cron() drains the subscription_manager_remote_sync queue with a hand-rolled claimItem()/deleteItem()/releaseItem() loop. Core's QueueWorker plugin system exists precisely for this: it drains declared queues during cron on its own, with standard retry semantics, and would shrink cron() to just the subscription-expiry sweep.

A hooks class should stay a thin dispatch layer, so the helper's destination is the service layer rather than SubscriptionManagerHooks.

Proposed resolution

1. Add SubscriptionManagerService::syncRemoteSubscriptions(UserInterface $account): void containing the helper's logic, with its dependencies injected rather than reached via \Drupal:: statics. Inject the service into SubscriptionManagerHooks (autowiring is already enabled for that class) and update userInsert() to call it.
2. Delete _subscription_manager_sync_remote_subscriptions() from the .module file. The underscore prefix marks it module-internal and it has no callers outside the module, so no deprecation cycle is needed. It is not a hook, so no #[LegacyHook] shim applies.
3. Add a QueueWorker plugin for subscription_manager_remote_sync (cron time declared in the plugin definition) that loads the user and calls the new service method, throwing to trigger core's standard release-and-retry handling. Remove the manual drain loop from cron(), leaving only the expiry sweep.
4. Port the existing kernel coverage (LoginRemoteSyncDeferralTest) to exercise the QueueWorker path, and add a unit or kernel test for the new service method directly.

If review prefers, item 3 can split into its own issue; it is included here because it touches the same call path and the two changes review most naturally together.

Remaining tasks

Sequence against related work: land after the #3603421: Automated Drupal 12 compatibility fixes for subscription_manager 1.0.x-dev hook conversion (this builds on the hooks class), and coordinate with #3616770: Self-healing remote subscription sync only queries the default connector, which rewrites this same routine to iterate all installed connectors instead of only the default. Doing this refactor first gives that issue a clean, injectable, testable home for its logic; doing them together in one branch is also reasonable.

User interface changes

None.

API changes

New public method SubscriptionManagerService::syncRemoteSubscriptions(). Removal of the module-internal procedural helper (underscore-prefixed, no external callers, no BC concern). New QueueWorker plugin; the queue name is unchanged, so already-enqueued items drain normally after deployment.

Data model changes

None.