storcon: exclude non-Active tenants from shard autosplitting (#9743)

## Problem

We didn't have a neat way to prevent auto-splitting of tenants. This
could be useful during incidents or for testing.

Closes https://github.com/neondatabase/neon/issues/9332

## Summary of changes

- Filter splitting candidates by scheduling policy

storage_controller/src/service.rs

@@ -6356,6 +6356,19 @@ impl Service {
 
         // Pick the biggest tenant to split first
         top_n.sort_by_key(|i| i.resident_size);
+
+        // Filter out tenants in a prohibiting scheduling mode
+        {
+            let locked = self.inner.read().unwrap();
+            top_n.retain(|i| {
+                if let Some(shard) = locked.tenants.get(&i.id) {
+                    matches!(shard.get_scheduling_policy(), ShardSchedulingPolicy::Active)
+                } else {
+                    false
+                }
+            });
+        }
+
         let Some(split_candidate) = top_n.into_iter().next() else {
             tracing::debug!("No split-elegible shards found");
             return;