MWPS merges immediately if the pipeline succeeded

What does this MR do?

When passing MWPS to the merge API, we should merge the MR immediately when the pipeline already succeeded.

On the other hand, we should not merge the MR when passing MWPS and the pipeline failed, even if the project allows merging with a failing pipeline.

Fixes #121620 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.