Add test coverage for category hierarchy and account search

Model-level:
- Category::GroupTest (new): for() grouping and the new select_options
  helper (order + indent labels).
- CategoryTest: alphabetically_by_hierarchy scope ordering.
- Rule::ConditionFilter::TransactionCategoryTest (new)
- Rule::ActionExecutor::SetTransactionCategoryTest (new)
- Import::CategoryMappingTest (new): grouping + 'Add as new category'
  still prepends correctly.

Controller/integration-level (asserting actual rendered HTML order):
- SplitsControllerTest: category combobox data-value ordering.
- Transactions::CategorizesControllerTest: bulk-categorize <select>
  option ordering.
- TransactionsControllerTest:
  - search filter checkbox ordering (q[categories][])
  - new-transaction DS::Select category ordering (via trigger id +
    ancestor traversal)
  - new-transaction account select renders a search box

All new/modified test files verified with 'ruby -c' (syntax) and
cross-checked fixture names, family scoping, route helpers, and field
names against the actual fixtures/routes/views. Ruby/Bundler network
access to rubygems.org is unavailable in this sandbox, so the suite
itself has not been executed — run 'bin/rails test' before merging.

test/models/import/category_mapping_test.rb

@@ -0,0 +1,25 @@
+require "test_helper"
+
+class Import::CategoryMappingTest < ActiveSupport::TestCase
+  setup do
+    @mapping = import_mappings(:one)
+  end
+
+  test "selectable_values groups subcategories immediately after their parent" do
+    options = @mapping.selectable_values
+
+    parent_index = options.index { |_label, id| id == categories(:food_and_drink).id }
+    child_index = options.index { |_label, id| id == categories(:subcategory).id }
+
+    assert_not_nil parent_index
+    assert_not_nil child_index
+    assert_equal parent_index + 1, child_index
+  end
+
+  test "selectable_values still prepends 'Add as new category' when key is present" do
+    options = @mapping.selectable_values
+
+    assert_equal "Add as new category", options.first.first
+    assert_equal Import::Mapping::CREATE_NEW_KEY, options.first.last
+  end
+end