Expose token_expires_at in runner details API

What does this MR do and why?

Expose the token_expires_at field in the GET /runners/:id (retrieve runner details) API response.

The field was already stored on the ci_runners table and returned by:

• POST /user/runners (create runner)
• POST /runners/:id/reset_authentication_token (reset token)

However, it was missing from GET /runners/:id, making it impossible for API consumers to inspect the token expiry of an existing runner without resetting the token.

The fix adds a single expose call to API::Entities::Ci::RunnerDetails, which is the entity used by the details endpoint.

Closes #627823 (closed)

References

• Issue: #627823 (closed)
• Retrieve runner's details API: https://docs.gitlab.com/api/runners/#retrieve-runners-details

Screenshots or screen recordings

Not applicable - API-only change.

Before	After
GET /runners/:id response does not include token_expires_at	GET /runners/:id response includes token_expires_at (null when not set)

How to set up and validate locally

1. Create a runner with a token expiry via the Users API.
2. Note the runner id from the response.
3. Retrieve the runner's details: GET /api/v4/runners/:id
4. Confirm token_expires_at is present in the response.

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