Draft: Add RubyGems toggles to the group and package-list UI

What

Adds the RubyGems frontend surfaces — the group Packages & registries forwarding toggle and the Package Registry list-page forwarding notice — alongside npm/PyPI/Maven.

Slice	MR
Setting (data + API)	!244621 (merged)
Forwarding + admin UI	!244623 (closed) — this MR is stacked on it
Group + package-list UI (frontend)	this MR

Why the UI ships after the capability

The setting and forwarding must exist before we expose user controls — a toggle for an inert setting is misleading. This frontend slice sits on top of the forwarding MR so every RubyGems forwarding control appears only once forwarding actually works.

Changes

• Group "Package forwarding" settings: RubyGems "Forward…" + "Enforce…" toggles (constants, component rubygemsMutation, PackageSettingsFields fragment, update mutation)
• Package Registry list page: RubyGems added to forwardingFieldToPackageTypeMapping + the GroupPackageSettings fragment, so the "request forwarding" notice covers RubyGems
• Consumes the rubygemsPackageRequestsForwarding GraphQL fields added in !244621 (merged)

TODO before ready

• Frontend specs for the RubyGems case: packages_forwarding_settings_spec.js, packages_list_spec.js (+ any GraphQL fixture). Draft until these land.

Manual GDK testing

1. Group → Settings → Packages and registries → Package forwarding: the RubyGems row appears next to npm/PyPI/Maven; toggle + Save persists to namespace_package_settings.
2. Enforce the setting at the instance (or a parent group) → the group's RubyGems control shows the inherited value and is disabled (greyed).
3. Package Registry list page: with RubyGems forwarding on, the request-forwarding notice covers RubyGems package searches.

Related to #605490.