Add artifact registry repository listing and write methods

What does this MR do and why?

Completes the repository CRUD surface of the ee/lib Artifact Registry client, on top of the merged step 1 foundation (!244245 (merged)):

• Client#repositories fetches the collection endpoint with filtering, sorting, and pagination arguments forwarded as query parameters, and returns a RepositoryPage value object. The opaque next/prev cursors are parsed from the RFC 8288 Link response header; only the cursor query-parameter value of each link is exposed, and a missing or partial Link header tolerantly yields nil cursors. A header that is present but carries none of the relations the client reads logs, rather than silently ending pagination.
• Client#create_repository, Client#update_repository, and Client#delete_repository complete the mutations. Optional fields are compacted out of the request bodies. Delete is idempotent: a 404 returns true, which also covers the retry race where the first attempt times out after deleting and the retry sees a 404. POST and PATCH are never retried.
• The request primitive returns the raw response (the listing needs the Link header), with each public method extracting the body shape it expects via success_body(slug:, expected:).
• The blank and dot-segment path guards are extracted into a shared guard_segments! helper and applied to every new endpoint, including the name that create_repository sends, so the traversal protections from the step 1 review carry over.
• An error body in an unrecognized JSON shape contributes only the values of a small allowlist of conventional human-readable keys (detail, message, title), rather than a dump of the whole object. The AR error envelope forbids additional properties on every error path, so a JSON body without an error key came from an intermediary such as an auth proxy, whose fields the client cannot vouch for.

This MR combines the previously separate step 2 (listing) and step 3 (write methods, !245812 (closed), now closed) into a single MR, to reduce review round trips and speed up landing.

Client plan (steps 2 and 3): https://gitlab.com/gitlab-org/ops/artifact-registry/-/blob/main/docs/plans/monolith/2026-07-02-ar-ruby-client.md

Related to #605078 (closed) and #605079 (closed).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist.

How to set up and validate locally

The client has no callers yet. From a Rails console with the AR service running locally on the GDK:

client = ArtifactRegistry::Client.new(current_user: User.first, token_exchange: Class.new { def token_for(_user, _slug) = 'dev-token' }.new)
client.repositories(slug: 'my-org')
client.create_repository(slug: 'my-org', name: 'demo', format: 'maven')
client.delete_repository(slug: 'my-org', name: 'demo')