GitLab Integration: Unable to fetch commits

### Self-Hosted Version

25.12.1

### CPU Architecture

x86_64

### Docker Version

29.0.2

### Docker Compose Version

2.40.3

### Machine Specification

- [x] My system meets the minimum system requirements of Sentry

### Installation Type

Upgrade but seems unrelated to any version

### Steps to Reproduce

Enable the GitLab integration and set it up according to the stepper. Create a release and wait for the background jobs to complete.

### Expected Result

Sentry Release is linked to a git tag/commit hash.

### Actual Result

We're receiving an error mail and in the logs i've found this:

```
09:30:01 [INFO] sentry.integrations.utils.metrics: integrations.slo.halted (organization_id=1 user_id=1 repository='thevea / Applikation' provider='integrations:gitlab' end_sha='6e72225f70fb93f79d8682a23f799e92924897cf' start_sha='5d6840b7f51aeef03a57b4c58fb20b9c07ed27a4' integration_domain='source_code_management' integration_name='gitlab' interaction_type='compare_commits' exception_summary="**IntegrationError('Gitlab requires an integration id.')**")
09:30:02 [INFO] sentry.mail: mail.queued (message_type='generic' release_id=202342 repository_id=2 message_id='<****>' message_to=('***sysadmin***',))
```

Unable to Fetch Commits

We were unable to fetch the commit log for your release (c5cbf581ed4c6f761df4ef684895d60401dc348f) for repository thevea / Applikation in the thevea organization due to the following error:

An internal error occurred with the integration and the Sentry team has been notified

### Event ID

_No response_

Start by tracing the GitLab integration's compare_commits path from the logged “Gitlab requires an integration id.” error, then inspect how release background jobs obtain the integration. Confirm the fix by reproducing the release flow and verifying that the release links to its git tag or commit without the Unable to Fetch Commits error.

Written by the indexing model from the issue text.