GitLab implementation needs to look up MR IID from commit SHA for apply to report status

### TL;DR

`CI_MERGE_REQUEST_IID` is not available on `push` events.

v3.0.0-alpha2

### Detailed design

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### Alternatives considered

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### Additional information

_No response_

The issue mentions that CI_MERGE_REQUEST_IID is not available on push events in GitLab. Look at the existing GitLab integration code in the repository, likely under a directory like 'internal/gitlab' or 'pkg/gitlab'. Find where commit statuses are reported and understand how to query the GitLab API to get the merge request IID from a commit SHA. Check for existing tests around GitLab functionality to see the expected behavior.

Written by the indexing model from the issue text.