storcon: skip offline nodes in get_top_tenant_shards (#12057)

## Summary

The optimiser background loop could get delayed a lot by waiting for
timeouts trying to talk to offline nodes.

Fixes: #12056

## Solution

- Skip offline nodes in `get_top_tenant_shards`

Link to Devin run:
https://app.devin.ai/sessions/065afd6756734d33bbd4d012428c4b6e
Requested by: John Spray (john@neon.tech)

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: John Spray <john@neon.tech>

storage_controller/src/service.rs

@@ -8538,8 +8538,9 @@ impl Service {
         Some(ShardCount(new_shard_count))
     }
 
-    /// Fetches the top tenant shards from every node, in descending order of
-    /// max logical size. Any node errors will be logged and ignored.
+    /// Fetches the top tenant shards from every available node, in descending order of
+    /// max logical size. Offline nodes are skipped, and any errors from available nodes
+    /// will be logged and ignored.
     async fn get_top_tenant_shards(
         &self,
         request: &TopTenantShardsRequest,
@@ -8550,6 +8551,7 @@ impl Service {
             .unwrap()
             .nodes
             .values()
+            .filter(|node| node.is_available())
             .cloned()
             .collect_vec();