Add availableQuickActions to Issue and WorkItem GraphQL types

What does this MR do and why?

Follow-up to !243123 (merged), which added MergeRequest.availableQuickActions. This MR exposes the same field on the Issue and WorkItem GraphQL types, so API clients can fetch the server-resolved quick actions set for issues and work items (including group-level work items) instead of hand-maintaining a command list.

No new logic is introduced: Resolvers::Notes::AvailableQuickActionsResolver already operates on any noteable. It resolves the container via Noteable#resource_parent, so project-level noteables evaluate against their project and group-level work items against their group — the same container Groups::AutocompleteService#commands already uses to back the web editor's /-autocomplete for group-level noteables.

As with the merge request field, each command's availability is resolved server-side via QuickActions::InterpretService#available_commands, so the returned set already accounts for:

• the current user's permissions,
• the noteable's state (for example /reopen only when closed),
• the namespace's licensed features (for example /weight, /iteration on the relevant plans; group-level work items return no commands without the license).

New fields

• Issue.availableQuickActions: [QuickActionCommand!]
• WorkItem.availableQuickActions: [QuickActionCommand!]

Both reuse the existing QuickActionCommand type (name, aliases, description, params, warning, icon).

Example query

{
  namespace(fullPath: "gitlab-org") {
    workItem(iid: "1") {
      availableQuickActions {
        name
        aliases
        description
        params
      }
    }
  }
}

Screenshots or screen recordings

Not applicable — GraphQL field, covered by the generated reference docs and specs.

How to set up and validate locally

1. In the GraphiQL explorer (/-/graphql-explorer), run the query above against a work item (or project { issue(iid:) { availableQuickActions { name } } } for an issue) you can access.
2. Confirm the returned commands match the /-autocomplete shown in that noteable's comment editor (state- and permission-dependent).

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.
• Documentation (GraphQL reference) regenerated.
• Tests added for the new resolver entry points (issue, project-level and group-level work items).