[elastic/elasticsearch] Fix issue #156237: Flaky test: ChangePointAggregatorTests.testSlopeUp

Job description

Fix GitHub issue 156237 in elastic/elasticsearch: https://github.com/elastic/elasticsearch/issues/156237 Repo: https://github.com/elastic/elasticsearch Target: Ship a focused upstream code fix or implementation for the linked issue. Required deliverables: A real code change upstream against elastic/elasticsearch. An upstream pull request linked to issue 156237. Testing notes covering the relevant fix or regression coverage. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL https://github.com/elastic/elasticsearch/issues/156237. Do not submit analysis only. Submit code plus the upstream PR.