fix(gateway): treat missing backend TLS secret as pending

[Dav-14]

When a backend module has not yet provisioned its TLS Secret, the Gateway
deployment reconciler fetched the Secret and returned any error as a hard
failure. A NotFound during this race surfaced the Gateway as errored rather
than pending.

Return core.NewPendingError() on apierrors.IsNotFound so the framework treats
it as pending (matching how other 'not ready yet' conditions are handled) and
retries. All other Get errors remain hard errors.

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[NumaryBot]

✅ Approve — automated review

The change correctly handles missing backend TLS secrets by converting Kubernetes NotFound errors into pending (non-retrying) errors while preserving hard-failure propagation for all other error types. The reconciliation watch mechanism (via the GatewayBackendTLSSecretLabel) ensures the controller re-enqueues correctly when the secret is eventually created, so no self-requeue is needed. Unit tests cover the missing, present, and non-NotFound error cases. The prior inline discussion confirmed correct watch-driven retry behavior and was resolved by the PR author. No actionable issues were identified by reviewers.

No findings.

[flemzord]

Looks good to me.