Fix/7220 preserve eval case order

[boopathi-376]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: [Bug]: Updating an eval case moves it to the end in Local/GCS eval sets #7220

Problem:

Updating an existing eval case moves it to the end of EvalSet.eval_cases in LocalEvalSetsManager.

Solution:

Replace the existing eval case in place instead of removing and appending it, preserving its original position.

Added a regression test to verify that updating a case does not change the order of eval cases.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

pytest tests/unittests/evaluation/ — 874 passed

Manual End-to-End (E2E) Tests:

Not applicable. The change is covered by unit tests.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective.
• New and existing unit tests pass locally.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Small, focused bug fix. The change is limited to the eval-set update helper and its regression test.