Become candidate on publication failure (#96490)

If the publication fails before starting (e.g. due to one of the precondition checks in `CoordinationState#handleClientValue`) then we must explicitly stand down as leader. Closes #96273

Author: DaveCTurner

docs/changelog/96490.yaml

@@ -0,0 +1,6 @@
+pr: 96490
+summary: Become candidate on publication failure
+area: Cluster Coordination
+type: bug
+issues:
+ - 96273

server/src/main/java/org/elasticsearch/cluster/coordination/Coordinator.java

@@ -1548,6 +1548,9 @@ assert getLocalNode().equals(clusterState.getNodes().get(getLocalNode().getId())
  followersChecker.setCurrentNodes(publishNodes);
  lagDetector.setTrackedNodes(publishNodes);
  publication.start(followersChecker.getFaultyNodes());
+ } catch (Exception e) {
+ assert currentPublication.isEmpty() : e; // should not fail after setting currentPublication
+ becomeCandidate("publish");
  } finally {
  publicationContext.decRef();
  }