Artifact Registry: landing route should explain a non-read-serving registry instead of redirecting

Context

Follow-up from !251192 (merged) (S10 Step 9, activation landing route), deferred per review by @rchanila.

To keep that MR small, the organization Artifact Registry landing route (/o/:organization_path/-/artifact_registry) now redirects any resolved registry to the slug-scoped repositories route, including non-read-serving statuses such as disabled, blocked, deleted, purged, and unrecognized values.

Problem

A non-read-serving registry (for example disabled) does not serve reads, so redirecting the user to the repositories app lands them on its native empty/not-found state rather than a clear explanation of the registry's status. There is no redirect loop (the repositories route renders its own state, it does not bounce back), but the experience is a dead end without context.

Proposed fix

Give the landing route a dedicated informational state for a resolved-but-non-read-serving registry: name the status and, for a viewer who can update_organization, link to the Activation settings section. This restores the branch that was removed from !251192 (merged).

Notes

• Behind the dark artifact_registry_ui feature flag; not user-visible yet.
• Should be closed before the flag flips on.