SOLR-17550: preferred Overseer might not work

[r4mercur]

add testcase to identify wrong method behaviour of setPreferredOverseer() in ZkController

https://issues.apache.org/jira/browse/SOLR-17550

Description

Like described in the ticket from the reporter: "By code inspection, ZkController.setPreferredOverseer creates a command message improperly. The key "node" should exist with the node name as value but instead the key is itself the node name – a bug introduced by a refactoring in 2022.
Ideally we understand the ramifications; like shouldn't this be found by a test?"

I only added the test case currently, not sure if the bug should also be fixed by using "node" as static string as key ?

Solution

My approach is a new unittest-case to identify the wrong behaviour of the setter method in the ZkController. I used the LLM "Claude Sonet 4.5" for problem description and solution proposals.

Tests

The new unittest-case written should identify problems, when using "getNodeName()" to set the key of the properties.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide
• I have added a changelog entry for my change