Honor download_mode in runner artifact downloads

What does this MR do and why?

The GET /api/v4/jobs/:id/artifacts endpoint did not declare download_mode, so client_requested_download_mode ignored the runner's requested transfer mode. This made an explicit download_mode=direct request fall back to the configured default, even when administrators allowed direct downloads.

This declares the optional download_mode parameter and lets an explicit direct request enable direct delivery for this endpoint. The shared download-mode resolver still enforces allowed_download_modes; requests without the parameter retain the existing behavior. Request specs cover direct mode allowed, direct mode disallowed, and the parameter absent.

References

• Required for gitlab-runner!7300 (merged)
• Relates to #624081 (closed)
• Related design and implementation: !252350 (merged)
• Review comment: gitlab-runner!7300 (comment 2678234098)

Screenshots or screen recordings

Not applicable: this is an API-only change with no visible UI.

How to set up and validate locally

Run the focused request specs:

bin/rspec spec/requests/api/ci/runner/jobs_artifacts_spec.rb -e 'GET /api/v4/jobs/:id/artifacts'

MR acceptance checklist

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