bug: sync_period 0 stops API7 synchronization events and retries

Problem

API7 Ingress Controller 2.1.0 defaults the api7ee provider to sync_period: 0s. In api7eeProvider.Start, startup synchronization runs, but the method then returns when SyncPeriod < 1 instead of entering the event loop.

Consequently, a zero period disables more than the periodic ticker:

• syncCh notifications are no longer consumed. For example, GatewayProxy configuration updates can update the cache and call syncNotify without applying the change to the gateway.
• retry notifications are no longer consumed.
• startup and delayed-startup synchronization failures do not reliably enter the retry path.

Expected behavior

sync_period: 0s should disable only periodic full synchronization. Explicit synchronization events, retries, and cancellation handling should remain active.

Additional lifecycle risk

The delayed startup synchronization currently uses time.AfterFunc, so it can run concurrently with the main synchronization loop or after Start returns. Synchronization, status handling, and retrier state changes should remain serialized in one loop.

Reference implementation

PR #457 contains a tested candidate fix and regression coverage, but is being closed so the implementation can be owned and reviewed by Ingress Controller engineering.

Validation completed on that patch:

• go test -v ./internal/provider/api7ee
• go test ./internal/provider/...
• go test -race ./internal/provider/api7ee

Start at api7eeProvider.Start and review PR #457's tested candidate and regression coverage. Run go test -v ./internal/provider/api7ee, go test ./internal/provider/..., and go test -race ./internal/provider/api7ee; done means sync_period 0 disables only periodic sync while events, retries, cancellation, and delayed startup remain serialized.