Pin kubernetes-client-api plugin to fix agent launch NoSuchMethodError

kubernetes:4353.vb_47977da_9417 requires kubernetes-client-api >=
7.3.1-256.v788a_0b_787114 (confirmed via plugins.jenkins.io), but that
plugin was never explicitly pinned in installPlugins — left to
resolve on its own at image-build time, it landed on an older,
incompatible version. Every build agent launch then crashed with
NoSuchMethodError: ConfigBuilder.withMasterUrl(String) inside
Reaper.preLaunch -> KubernetesCloud.connect ->
KubernetesFactoryAdapter.createClient: pods provisioned fine at the
Kubernetes API level, but the controller could never actually connect
an agent to them, so every demo-go-app build hung forever at "Still
waiting to schedule task".

helm-overrides/k8s-admin-prd-ase1/jenkins/custom-values.yaml

@@ -23,8 +23,21 @@ jenkins:
     # above. Lists replace wholesale in Helm, not merge, so this is the
     # chart's full default list with just that one version corrected —
     # not a hand-picked subset.
+    #
+    # kubernetes-client-api added explicitly (not part of the chart's
+    # default list) — kubernetes:4353.vb_47977da_9417 requires
+    # kubernetes-client-api >= 7.3.1-256.v788a_0b_787114 (confirmed via
+    # https://plugins.jenkins.io/kubernetes/dependencies/), but left
+    # unpinned it resolved to an older version at image-build time,
+    # producing `NoSuchMethodError:
+    # ConfigBuilder.withMasterUrl(String)` on every agent launch attempt
+    # — pods provisioned fine but the controller crashed trying to
+    # actually connect the agent (Reaper.preLaunch -> KubernetesCloud.connect
+    # -> KubernetesFactoryAdapter.createClient), so builds hung forever at
+    # "Still waiting to schedule task".
     installPlugins:
       - kubernetes:4353.vb_47977da_9417
+      - kubernetes-client-api:7.3.1-256.v788a_0b_787114
       - workflow-aggregator:608.v67378e9d3db_1
       - git:5.7.0
       - configuration-as-code:2006.v001a_2ca_6b_574