storcon: respect tenant scheduling policy in drain/fill (#9657)

## Problem

Pinning a tenant by setting Pause scheduling policy doesn't work because
drain/fill code moves the tenant around during deploys.

Closes: https://github.com/neondatabase/neon/issues/9612

## Summary of changes

- In drain, only move a tenant if it is in Active or Essential mode
- In fill, only move a tenant if it is in Active mode.

The asymmetry is a bit annoying, but it faithfully respects the purposes
of the modes: Essential is meant to endeavor to keep the tenant
available, which means it needs to be drained but doesn't need to be
migrated during fills.

storage_controller/src/service.rs

@@ -6721,6 +6721,16 @@ impl Service {
             .tenants
             .iter_mut()
             .filter_map(|(tid, tenant_shard)| {
+                if !matches!(
+                    tenant_shard.get_scheduling_policy(),
+                    ShardSchedulingPolicy::Active
+                ) {
+                    // Only include tenants in fills if they have a normal (Active) scheduling policy.  We
+                    // even exclude Essential, because moving to fill a node is not essential to keeping this
+                    // tenant available.
+                    return None;
+                }
+
                 if tenant_shard.intent.get_secondary().contains(&node_id) {
                     if let Some(primary) = tenant_shard.intent.get_attached() {
                         return Some((*primary, *tid));