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.

[epugh]

@iprithv if you can confirm my changes to your PR, then I'll merge ;-)

[iprithv]

LGTM, thanks @epugh for finishing off the migration.. good catch on the endpoint shadow and the 500→400 standalone fix.. admin UI cleanup is a nice bonus :)

[epugh]

Thank you @iprithv for this. I wanted to layout more tickets to help people, and so I have this ticket https://issues.apache.org/jira/browse/SOLR-18425 with six subtasks ;-)

[dsmiley]

10x backport problem: c6d595b (SOLR-16390: Migrate list cluster nodes API to JAX-RS #4865) conflicts because its prerequisite is missing: ListClusterNodesResponse.java and the V2 ListClusterNodes API model were introduced on main by #4154 ("Migrate org.apache.solr.cli tools from V1 to V2 APIs"), which has NO milestone and is NOT on branch_10x. PR #4865's own milestone is 10.x.