Self-healing remote subscription sync only queries the default connector

Problem/Motivation

_subscription_manager_sync_remote_subscriptions() exists to self-heal the case where a remote subscription contract exists but the local subscription entity is missing (for example a webhook that never fired). It runs on hook_user_insert() and when cron drains the subscription_manager_remote_sync queue populated at login.

The function only ever queries the connector named in default_connector. On a site with more than one connector, a missed webhook on any non-default connector is never reconciled: the user's remote contract is active, but no local entity is created, no roles are granted, and every login re-enqueues a sync job that checks the wrong service. This contradicts the multi-connector design of the rest of the module, where syncPlans() already iterates all installed connector definitions.

Steps to reproduce

1. Install connectors A and B; set default_connector to A.
2. Create a remote subscription on connector B for an email address with no local subscription entity (simulating a missed webhook).
3. Register or log in as that user and run cron.
4. No local subscription is created; connector B was never consulted.

Proposed resolution

Iterate all installed connector definitions, mirroring SubscriptionManagerService::syncPlans():

• Keep the cheap local short-circuit (an active local subscription skips all remote calls).
• For each installed connector, call loadRemoteSubscriptionsByUser() and sync any results via syncRemoteSubscriptionToLocal(), wrapping each connector in its own try/catch so one provider's outage doesn't block reconciliation against the others.
• Optionally stop after the first connector that yields results. This short-circuit is only valid if the single-active-subscription invariant from #3616778: Define and enforce single-active-subscription semantics (loadLocalSubscription() returns an arbitrary first match) is adopted (its Option A); if concurrent subscriptions are allowed there, every connector must always be consulted.

Remaining tasks

• Sequence against #3616880: Move _subscription_manager_sync_remote_subscriptions() into SubscriptionManagerService and convert the cron queue drain to a QueueWorker plugin, which moves this routine out of the .module file into SubscriptionManagerService::syncRemoteSubscriptions() with injected dependencies and a QueueWorker plugin. Landing that refactor first gives this fix a clean, testable home; alternatively both can be done in one branch there.
• Patch, plus a kernel test with two test connectors (one returning a remote subscription, one not; assert both are consulted and the entity is created with the correct connector_plugin_id).
• Change record if the stop-after-first behavior is adopted.

User interface changes

None.

API changes

None if implemented in the internal helper. If #3616880: Move _subscription_manager_sync_remote_subscriptions() into SubscriptionManagerService and convert the cron queue drain to a QueueWorker plugin lands first, the change lands in the public SubscriptionManagerService::syncRemoteSubscriptions() method instead, still with no signature change.

Data model changes

None.