Omit inactive users from board mention picker

The board-scoped mention prompt was returning all users with board access,
including those marked as inactive. This adds the `.active` scope to match
the behavior of the account-wide mention prompt.

Fixes https://app.fizzy.do/5986089/cards/3861

test/controllers/prompts/boards/users_controller_test.rb

@@ -11,4 +11,16 @@ class Prompts::Boards::UsersControllerTest < ActionDispatch::IntegrationTest
     assert_response :success
     assert_select "lexxy-prompt-item", count: 3
   end
+
+  test "index excludes inactive users" do
+    get prompts_board_users_path(@board)
+    assert_response :success
+    assert_select "lexxy-prompt-item[search*='David']", count: 1
+
+    users(:david).update!(active: false)
+
+    get prompts_board_users_path(@board)
+    assert_response :success
+    assert_select "lexxy-prompt-item[search*='David']", count: 0
+  end
 end