feat(runtime): include leases in runtime diagnostics

This change updates `Runtime::spawn_lease` (added in b26b009) to register leases
with the admin server's diagnostics endpoint when the `runtime-diagnostics`
feature is enabled:

```json
{
  "initialTimestamp": "2025-01-04T19:28:33Z",
  "currentTimestamp": "2025-01-04T19:29:28Z",
  "leases": [
    {
      "name": "foobar",
      "namespace": "default",
      "claimant": "code",
      "fieldManager": "kubert-examples",
      "leaseDurationSeconds": 30,
      "renewGracePeriodSeconds": 1,
      "updates": 2,
      "creationTimestamp": "2025-01-04T19:28:33Z",
      "lastUpdateTimestamp": "2025-01-04T19:29:24Z",
      "resourceVersion": "1822330",
      "current": {
        "holder": "code",
        "expiry": "2025-01-04T19:29:54.207379734Z"
      }
    }
  ]
}
```

kubert/src/admin/diagnostics/lease.rs

@@ -15,23 +15,22 @@ pub(super) struct LeaseState {
     name: String,
     namespace: String,
     claimant: String,
+    field_manager: Cow<'static, str>,
     lease_duration_seconds: f64,
     renew_grace_period_seconds: f64,
-    field_manager: Cow<'static, str>,
-    #[serde(skip_serializing_if = "Option::is_none")]
-    claim: Option<crate::lease::Claim>,
+    #[serde(flatten)]
+    stats: LeaseStats,
     #[serde(skip_serializing_if = "Option::is_none")]
     resource_version: Option<String>,
-    stats: LeaseStats,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    current: Option<crate::lease::Claim>,
 }
 
 #[derive(Clone, Debug, serde::Serialize)]
 #[serde(rename_all = "camelCase")]
 struct LeaseStats {
-    creation_timestamp: Time,
-
     updates: u64,
-
+    creation_timestamp: Time,
     #[serde(skip_serializing_if = "Option::is_none")]
     last_update_timestamp: Option<Time>,
 }
@@ -59,7 +58,7 @@ impl LeaseDiagnostics {
             field_manager: field_manager.clone().unwrap_or(Cow::Borrowed(
                 crate::lease::LeaseManager::DEFAULT_FIELD_MANAGER,
             )),
-            claim: None,
+            current: None,
             resource_version: None,
             stats: LeaseStats {
                 creation_timestamp: now,
@@ -79,13 +78,13 @@ impl LeaseDiagnostics {
         resource_version: String,
     ) {
         let mut state = self.0.write();
-        if claim.as_deref() == state.claim.as_ref()
+        if claim.as_deref() == state.current.as_ref()
             && Some(&*resource_version) == state.resource_version.as_deref()
         {
             return;
         }
         let now = Time(chrono::Utc::now());
-        state.claim = claim.as_deref().cloned();
+        state.current = claim.as_deref().cloned();
         state.resource_version = Some(resource_version);
         state.stats.updates += 1;
         state.stats.last_update_timestamp = Some(now);