Use `AgentTokensFinder` to find single agent tokens

What does this MR do and why?

The AgentTokensFinder was introduced in these MRs: !103170 (merged) & !104038 (merged) and used for fetching all agent tokens of an agent.

Here, the AgentTokensFinder class is used for fetching a single agent_token record in the GET /projects/:id/cluster_agents/:agent_id/tokens/:token_id endpoint. The FinderMethods module is included to support finding a single record.

Note that this follows the original behavior of returning the token regardless of status (active or revoked).

Issue: #363119 (closed)

Screenshots or screen recordings

Setup

Example project (ID=27):

Project Agent -> Tokens page

Project agent and agent tokens in the database

Test

Fetching a single agent token

Fetching a single agent token that is revoked

When specified agent_id is not under the specified project

When specified agent does not exist

How to set up and validate locally

Setup

1. Create a project (or use an existing one)
2. Set up an agent in that project
   • This will register an agent and create one agent token
3. Create more agent tokens:
   1. Go to project Infrastructure -> Kubernetes clusters page
   2. Go to the agent's page
   3. Select the Access tokens tab
   4. Click the Create token button

Validate

Use curl to test the API endpoint

1. Set up a project or personal access token to make sure your curl request is authorized

2. Run the following curl request:

   curl -ki -X GET \
   --header "Authorization: Bearer <your project or personal access token>" \
   "https://<local gitlab url>/api/v4/projects/<project_id>/cluster_agents/<agent_id>/tokens/<token_id>"

   Example:

   curl -ki -X GET \
   --header "Authorization: Bearer <token>" \
   "https://gdk.test:3443/api/v4/projects/27/cluster_agents/7/tokens/6"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.