Update merge-when-pipeline-succeeds behavior on API

What does this MR do?

When the merge_when_pipeline_succeeds param is passed in, an MR can be merged even when the pipeline has failed.

This change adds a check that prevents this from happening when attempting to merge through the API.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• [-] Code review guidelines

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

#26293 (closed)