Resolve "CI_MERGE_REQUEST_DIFF_BASE_SHA not updating on branch change"

``## What does this MR do and why?

This MR fixes an issue where CI_MERGE_REQUEST_DIFF_BASE_SHA was not updating correctly when the target branch changes in a merge request. The problem occurred because the merge_request_diff_for method was returning the first matching diff instead of the most recent one.

The fix adds a reverse parameter to the merge_request_diff_for method that changes the search order from first-to-last to last-to-first, ensuring the most recent diff is returned when multiple diffs match the same criteria. The Ci::Pipeline#merge_request_diff method now calls merge_request_diff_for with reverse: true to get the latest diff.

This ensures that CI variables like CI_MERGE_REQUEST_DIFF_BASE_SHA reflect the most recent diff's base SHA when the target branch changes or multiple diffs exist for the same commit SHA.``

References

Closes #442031 (closed)

How to set up and validate locally

1. Create a merge request with multiple diffs (e.g., by changing the target branch)
2. Verify that CI_MERGE_REQUEST_DIFF_BASE_SHA reflects the most recent diff's base SHA
3. Test that CI variables are updated correctly when the target branch changes

MR acceptance checklist

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