feat: add cluster name to leaf monitoring data

[joeriddles]

Add cluster name to leaf monitoring data.

This would be useful for downstream consumers of monitoring data and topology visualizations.

Signed-off-by: Joe Riddle joe@synadia.com

---

Tested locally with the following configs:

server_name: "hub"
listen: "127.0.0.1:4222"
http: "127.0.0.1:8222"

leafnodes {
  listen: "127.0.0.1:7422"
}

server_name: "leaf1"
listen: "127.0.0.1:4223"
http: "127.0.0.1:8223"

cluster {
  name: "leaf-cluster"
  listen: "127.0.0.1:6223"
}

leafnodes {
  remotes [{
    url: "nats-leaf://127.0.0.1:7422"
  }]
}

❯ curl -s http://127.0.0.1:8222/leafz | jq .leafs
[
  {
    "id": 5,
    "name": "leaf1",
+    "cluster": "leaf-cluster",
    "is_spoke": false,
    "account": "$G",
    "ip": "127.0.0.1",
    "port": 51218,
    "rtt": "154µs",
    "in_msgs": 0,
    "out_msgs": 0,
    "in_bytes": 0,
    "out_bytes": 0,
    "subscriptions": 5,
    "compression": "s2_uncompressed"
  }
]

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[chatgpt-codex-connector]

Hide synthetic leaf cluster names from /leafz

When a remote leaf has leafnodes.remotes and a server_name but no cluster block, Server.New intentionally fabricates opts.Cluster.Name = opts.ServerName only for internal bookkeeping (server/server.go:820-825), and /varz already strips that back out (server/monitor.go:1708-1711). leafNodeResumeConnectProcess still sends this synthetic value in the leaf CONNECT (server/leafnode.go:3367-3375, server/leafnode.go:976-978), so exposing ln.leaf.remoteCluster here makes /leafz report standalone leaves as if they belonged to a real cluster named after the server. Any topology consumer using /leafz will misclassify non-clustered leaves whenever server_name is configured.

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[joeriddles]

@wallyqs thanks for the review. I didn't mean to close this PR. Re-opened it.