Move latest-finished rollout selection to the backend

What does this MR do and why?

Environments with more than 20 rollout environments had a correctness problem. environment_card.vue fetched an environment's entire rolloutEnvironments connection (capped at 20 by the resolver) and picked the most recently finished one client-side via filter + reduce. If an environment had more than 20 rollout environments, the actual most-recently-finished one might not be among the 20 returned, so the card could show a stale release version, deploy time, or deployed-by user.

This MR moves the selection to the backend, where it belongs. Two new scopes (finished and most_recently_finished) are added to the Cd::RolloutEnvironment model, along with a latest_finished_per_environment class method (using a DISTINCT ON query, a pattern already used elsewhere in this codebase) that a GraphQL batch loader calls. A new latestFinishedRolloutEnvironment field is exposed on the CdEnvironment GraphQL type (batch-loaded, gated behind the existing ai_native_deploy feature flag, marked as an experimental field targeting milestone 19.4). The frontend query and component are simplified to read this single field directly, which also removes the now-unnecessary client-side filter+reduce logic.

This is not a visual or styling change — the same fields render the same way in the UI, just sourced correctly and more simply now.

References

• Follow-up to !244679 (merged) (non-blocking review suggestion from @anna_vovchenko)

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Enable the ai_native_deploy feature flag locally:

   Feature.enable(:ai_native_deploy)

2. Create data as per the MR description !244679 (merged)

3. Navigate to an organization's CD environments page.

4. Confirm the release, last-deploy, and deployed-by fields reflect the backend-selected latest finished rollout environment.

MR acceptance checklist

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