Update availability of mwcp

What does this MR do and why?

Updates the "Merge When Checks Pass" (MWCP) auto-merge availability check to only block when the pipeline has succeeded, rather than when the pipeline is not in progress.

Before: MWCP was unavailable when the merge request was mergeable AND the pipeline was not considered "in progress" (which includes various non-running states).

After: MWCP is only unavailable when the merge request is mergeable AND the pipeline has explicitly succeeded.

This change ensures more precise control over when the auto-merge option should be available.

Changes

• app/services/auto_merge/merge_when_checks_pass_service.rb: Changed condition from !diff_head_pipeline_considered_in_progress? to diff_head_pipeline_success?
• Updated corresponding specs to reflect the new behavior

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