Fix CI artifact sizes not logged for some runner endpoints

What does this MR do and why?

!88140 (merged) may have quietly broken the logging for the GET /api/:version/jobs/:id/artifacts endpoint. Since log_artifact_size was previously defined as a runner API helper and a general API helper, the runner API version won. This caused the lazy attribute loader to discard the artifact since it was a JobArtifactUploader type instead of Ci::JobArtifact.

We can fix this by renaming the method to log_artifact_file_size and adding a test to ensure the right type is called.

Relates to #369033 (closed)

How to set up and validate locally

1. Create a CI job with stages that pass artifacts:

image: alpine:latest

test:
  script:
    - mkdir -p test
    - echo "hello" > test/test.txt
  artifacts:
    paths:
      - test
  cache:
    paths:
      - test

deploy:
  stage: deploy
  script:
    - echo hello

2. Observe /var/log/gitlab/gitlab-rails/api_json.log has a meta.artifact_size:

$ grep 'GET /api/:version/jobs/:id/artifacts' api_json.log | jq | grep artifact_size
  "meta.artifact_size": 599,

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.