DEV: Single admin plugin page for consistent admin plugin UX (#26024)

This commit adds new plugin show routes (`/admin/plugins/:plugin_id`) as we move
towards every plugin having a consistent UI/landing page.

As part of this, we are introducing a consistent way for plugins
to show an inner sidebar in their config page, via a new plugin
API `register_admin_config_nav_routes`

This accepts an array of links with a label/text, and an
ember route. Once this commit is merged we can start the process
of conforming other plugins to follow this pattern, as well
as supporting a single-page version of this for simpler plugins
that don't require an inner sidebar.

Part of /t/122841 internally

spec/requests/admin/plugins_controller_spec.rb

@@ -39,4 +39,66 @@ RSpec.describe Admin::PluginsController do
       end
     end
   end
+
+  describe "#show" do
+    before do
+      spoiler_alert =
+        Plugin::Instance.parse_from_source(
+          File.join(Rails.root, "plugins", "spoiler-alert", "plugin.rb"),
+        )
+      poll =
+        Plugin::Instance.parse_from_source(File.join(Rails.root, "plugins", "poll", "plugin.rb"))
+
+      Discourse.stubs(:plugins_by_name).returns(
+        { "discourse-spoiler-alert" => spoiler_alert, "poll" => poll },
+      )
+    end
+
+    context "while logged in as an admin" do
+      before { sign_in(admin) }
+
+      it "returns a plugin" do
+        get "/admin/plugins/poll.json"
+
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["name"]).to eq("poll")
+      end
+
+      it "returns a plugin with the discourse- prefix if the prefixless version is queried" do
+        get "/admin/plugins/spoiler-alert.json"
+
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["name"]).to eq("spoiler-alert")
+      end
+
+      it "404s if the plugin is not found" do
+        get "/admin/plugins/casino.json"
+
+        expect(response.status).to eq(404)
+        expect(response.parsed_body["errors"]).to include(I18n.t("not_found"))
+      end
+    end
+
+    context "when logged in as a moderator" do
+      before { sign_in(moderator) }
+
+      it "returns plugins" do
+        get "/admin/plugins/poll.json"
+
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["name"]).to eq("poll")
+      end
+    end
+
+    context "when logged in as a non-staff user" do
+      before { sign_in(user) }
+
+      it "denies access with a 404 response" do
+        get "/admin/plugins/poll.json"
+
+        expect(response.status).to eq(404)
+        expect(response.parsed_body["errors"]).to include(I18n.t("not_found"))
+      end
+    end
+  end
 end