Extend Artifact Registry client with remote settings and a connection test

What does this MR do and why?

ArtifactRegistry::Client cannot send a remote repository's settings and cannot probe its upstream, so the remote create, edit, and test surfaces have no transport to call.

• create_repository and update_repository take a settings Hash and pass it through uninterpreted. The caller expresses the credential three-state through key presence inside that Hash: an absent credentials key leaves the stored values alone, and an explicit nil clears them. Neither method flattens a nested nil.
• Both methods refuse the two calls Artifact Registry rejects unconditionally, so a caller gets a message naming the reason rather than an opaque 400. create_repository raises when settings accompanies a kind other than remote, since an absent kind resolves to hosted, and when a remote create carries no settings. update_repository raises on an explicit nil settings, since a leading null in that key is refused on every request.
• test_upstream_connection posts to .../repositories/:name/test with no request body, because Artifact Registry answers 400 to a non-empty one, and returns the verdict as the new ArtifactRegistry::ConnectionTestResult value object.
• A 404 from the test route raises ApiError rather than resolving nil. That route exists on remote repositories only, so a hosted or virtual repository is a genuine not-found. The method therefore does not use the nil_on_missing helper that the read methods use.
• A 200 whose body carries no passed key raises UnavailableError. It would otherwise read as a probe that found the upstream unreachable, which is a verdict Artifact Registry did not give. ConnectionTestResult#passed reads only a boolean true as reachable, for the same reason.

Backend only. Nothing calls either addition yet, so there is no user-visible change. Every caller is gated by the artifact_registry_ui feature flag, which is default off. The client itself is not gated. No changelog entry, and no strings to externalize.

References

• Issue: #618512 (closed)
• Contract: POST /api/v1/{slug}/repositories/{repository_name}/test and the ConnectionTestResult schema in api/openapi/v1.yaml

How to set up and validate locally

There is no UI to drive. Run the specs:

bundle exec rspec ee/spec/lib/artifact_registry/client_spec.rb ee/spec/lib/artifact_registry/connection_test_result_spec.rb

MR acceptance checklist

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