[ML] Add validation for updating num_threads

[prwhelan]

num_threads cannot be updated and should result in a validation error

Fix #137129

---

{
  "error": {
    "root_cause": [
      {
        "type": "validation_exception",
        "reason": "Validation Failed: 1: [num_threads] cannot be updated;"
      }
    ],
    "type": "validation_exception",
    "reason": "Validation Failed: 1: [num_threads] cannot be updated;"
  },
  "status": 400
}

[elasticsearchmachine]

Hi @prwhelan, I've created a changelog YAML for you.

[Copilot]

Pull Request Overview

This PR adds validation to prevent updating the num_threads parameter in Elasticsearch internal service settings, addressing issue #137129. When users attempt to update this parameter, they will now receive a clear validation error instead of potentially experiencing undefined behavior.

Key changes:

• Added validation logic to reject num_threads updates in service settings
• Added test coverage to verify the validation error is thrown correctly
• Added changelog entry documenting this bug fix

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
ElasticsearchInternalServiceSettings.java	Added validation check to throw ValidationException when num_threads update is attempted
ElasticsearchInternalServiceTests.java	Added test case to verify num_threads update validation
docs/changelog/138122.yaml	Added changelog entry for this bug fix

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
✅	9.2
❌	8.19	Commit could not be cherrypicked due to conflicts
❌	9.1	Commit could not be cherrypicked due to conflicts

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 138122