fix stuck finalizer on delete

[evhen14]

Bug Fix

What Changed

When deleting a RestateDeployment, the finalizer no longer gets permanently
stuck with CleanupFailed(DeploymentInUse). Previously, cleanup_old_replicasets
treated the latest ReplicaSet's Restate deployment as an unconditional blocker
because it is always active = true in sys_service. Since no new version
ever registers during deletion, the active_count > 0 check caused an infinite
retry loop with no way out.

When rsd.deletion_timestamp.is_some(), active deployments are now scheduled
for drain (respecting spec.restate.drainDelaySeconds) rather than treated as
a permanent blocker. After the drain period they proceed through the existing
force-delete path.

Why This Matters

This is the common case in ephemeral PR/preview environments: a service is
deployed, registered with Restate, but no workflows are ever invoked before the
environment is torn down. Without this fix, deleting the RestateDeployment
would stall forever and require manual intervention to remove the finalizer.

Impact on Users

• Existing deployments being deleted: Stuck RestateDeployment objects will
  make progress on the next reconcile after upgrading.
• New deletions: Behave as expected — drain delay is respected, then the
  Restate deployment is force-deleted and the finalizer released.
• No migration required.

Related Issues

• Issue RestateDeployment finalizer stuck in CleanupFailed(DeploymentInUse) loop when no invocations ran #172: RestateDeployment finalizer stuck in CleanupFailed(DeploymentInUse) loop when no invocations ran

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