Correct the MR context commits endpoint

What does this MR do?

Rework the entities introduced in !23701 (merged)

I missed that ::API::Entities::CommitEntity had been renamed to ::API::Entities::Commit, and the endpoint was returning ::CommitEntity entities as a consequence.

This was just merged, so I think it's safe to pare this back to ::API::Entities::Commit, even though it has fewer fields. We can approach re-adding them later, if they're required.

I also add pagination, which was missing.

Screenshots

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.
• 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

Closes #199724 (closed)