feat(combobox): use single chevron-down icon in ComboboxTrigger

Aligns ComboboxTrigger with the base shadcn Combobox, which uses a single
chevron-down affordance. ComboboxInputTrigger already renders chevron-down;
this makes the default trigger consistent.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: pierry01

gem/lib/ruby_ui/combobox/combobox_trigger.rb

@@ -52,10 +52,7 @@ def icon
         stroke_linejoin: "round"
       ) do |s|
         s.path(
-          d: "m7 15 5 5 5-5"
-        )
-        s.path(
-          d: "m7 9 5-5 5 5"
+          d: "m6 9 6 6 6-6"
         )
       end
     end

gem/test/ruby_ui/combobox_test.rb

@@ -187,8 +187,7 @@ def test_combobox_badge_trigger_no_chevron
 
   def test_combobox_trigger_chevron_down
     output = phlex { RubyUI.ComboboxTrigger(placeholder: "Pick") }
-    assert_match(/m7 15 5 5 5-5/, output)
-    assert_match(/m7 9 5-5 5 5/, output)
+    assert_match(/m6 9 6 6 6-6/, output)
   end
 
   def test_combobox_trigger_sets_placeholder_data