Support fine-grained PATs for the go-get discovery endpoint

Jimmy Spagnolarequested to merge
gitlab-community/gitlab-org/gitlab:599476-go-get-granular-pat into master

What does this MR do and why?

Support fine-grained PATs for the go-get discovery endpoint

The ?go-get=1 meta-tag discovery request authorized the caller through Gitlab::Auth::Result#authentication_abilities, which is always empty for a fine-grained personal access token. Every request for a private project made with one returned 404, so go get could not be used with fine-grained tokens even though the subsequent git clone already supports them.

Authorize the token against its granular scopes, the same way Gitlab::GitAccess does. A fine-grained token now needs the download_code permission on the project, matching what the clone itself requires.

Closes #599476 (closed)

Co-Authored-By: Claude Opus 5 noreply@anthropic.com

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Related to #599476 (closed)

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