Enhance admin.hidden to support multi-tenant context: pass req and allow async

[antonioru]

Hey team,

In multi-tenant setups, when defining a Collection the admin.hidden function only receives the { user } argument, which makes it hard to drive visibility based on the active tenant or other request-scoped data.
A common need is to show or enable a collection only when the user is operating within an allowed tenant.
Without access to the current request, we can’t reliably read the active tenant selection (for example from a multi-tenant plugin) or perform context-aware lookups.
I propose extending admin.hidden so it can optionally receive the req object and support returning a promise, while preserving backward compatibility with the existing synchronous signature.
This would let implementations base visibility on request-scoped tenant info, cookies/headers, or quick async checks, and continue working for simple user-only cases.
Current API:

admin: {
  hidden: ({ user }) => boolean
}

Proposed (backward compatible):

admin: {
  // still works synchronously
  hidden: ({ user, req }) => boolean,

  // or asynchronously for tenant lookups / DB checks
  hidden: async ({ user, req }) => {
    const activeTenant = req?.tenantSelection?.tenant; // example
    return !user?.tenants?.includes(activeTenant?.id);
  },
}

Example: Hide a collection unless the user selects an active tenant:

export const Orders: CollectionConfig = {
  slug: 'orders',
  admin: {
    hidden: async ({ user, req }) => {
      const activeTenantId = req?.tenantSelection?.tenant?.id;
      return !activeTenantId || !user?.tenants?.some(t => t.id === activeTenantId);
    },
  },
};