Run Action endpoint: accept group + task instead of allocID

The `nomad job action` endpoint accepts either:
- job name, task name, group name, and action name
- job name, allocID, and action name

but the `/job/action` endpoint only accepts the latter.

In the event that multiple allocations are available to run an action on, the `nomad job action` endpoint currently selects one of the running ones. The API endpoint should do the same.

Start by comparing the `nomad job action` implementation with the `/job/action` API endpoint, focusing on how each resolves a target allocation. Confirm the endpoint's request handling and selection behavior, then verify that group and task inputs select a running allocation when multiple allocations are available.

Written by the indexing model from the issue text.