SOLR-16390: Migrate list cluster nodes API to JAX-RS

[iprithv]

https://issues.apache.org/jira/browse/SOLR-16390

Converts GET /api/cluster/nodes from the homegrown @EndPoint wrapper to JAX-RS, matching ClusterProperty / ListCollections.

• JAX-RS interface ListClusterNodesApi and typed ListClusterNodesResponse already existed in solr/api
• ListClusterNodes owns the live-node listing
• Path and response key nodes are unchanged
• SolrJ ClusterApi.ListClusterNodes
• HTTP coverage in SolrCloud (lists live nodes) and standalone (400 when not SolrCloud)

[epugh]

Looks like the old ClusterAPI.getNodes() @EndPoint never got removed — it's still shadowing the new JAX-RS class for real traffic. Confirmed by running ListClusterNodesStandaloneTest, which fails (500 instead of 400) because the request never reaches ListClusterNodes. #4866 removed the analogous legacy methods for the command-status migration — same fix needed here for getNodes.