Runner details API does not return token_expires_at field

Summary

The Create a runner linked to a user API accepts and stores a token_expires_at parameter when creating a runner authentication token. However, the Retrieve runner's details API (GET /api/v4/runners/:id) does not include token_expires_at in its response, making it impossible for API consumers to inspect the token expiry of an existing runner.

Steps to reproduce

1. Create a runner with a token expiry via the Users API:

   POST /api/v4/user/runners
   {
     "runner_type": "project_type",
     "project_id": <project_id>,
     "token_expires_at": "2025-01-01T00:00:00Z"
   }

2. Note the token_expires_at value in the response.
3. Retrieve the runner's details:

   GET /api/v4/runners/:id

4. Observe that token_expires_at is absent from the response.

Current behavior

GET /api/v4/runners/:id does not include token_expires_at in its response payload, even when the runner was created with a token expiry date.

Expected behavior

GET /api/v4/runners/:id should include token_expires_at in its response payload, consistent with the field being accepted on creation and already surfaced by the reset authentication token endpoint (POST /api/v4/runners/:id/reset_authentication_token).

Relevant links

• Create a runner linked to a user (accepts token_expires_at): https://docs.gitlab.com/api/users/#create-a-runner-linked-to-a-user
• Retrieve runner's details (missing token_expires_at): https://docs.gitlab.com/api/runners/#retrieve-runners-details

Environment

• Deployment type: GitLab.com and Self-Managed
• Affects: REST API