Add strict2_parse to increment and decrement tags

Both tags previously accepted any string as a variable name via
`markup.strip`. Now they use `parse_with_selected_parser` and validate
the variable name with `p.consume(:id)` in strict2 mode.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

test/integration/tags/increment_tag_test.rb

@@ -27,4 +27,50 @@ class IncrementTagTest < Minitest::Test
       '{%decrement starboard %}',
     )
   end
+
+  def test_increment_strict2_rejects_invalid_variable_name
+    assert_raises(Liquid::SyntaxError) do
+      Template.parse('{% increment foo bar %}', error_mode: :strict2)
+    end
+  end
+
+  def test_increment_strict2_rejects_variable_starting_with_number
+    assert_raises(Liquid::SyntaxError) do
+      Template.parse('{% increment 11aa %}', error_mode: :strict2)
+    end
+  end
+
+  def test_increment_strict2_accepts_valid_variable_name
+    template = Template.parse('{% increment my-var %}', error_mode: :strict2)
+    assert_equal('0', template.render)
+  end
+
+  def test_decrement_strict2_rejects_invalid_variable_name
+    assert_raises(Liquid::SyntaxError) do
+      Template.parse('{% decrement foo bar %}', error_mode: :strict2)
+    end
+  end
+
+  def test_decrement_strict2_rejects_variable_starting_with_number
+    assert_raises(Liquid::SyntaxError) do
+      Template.parse('{% decrement 11aa %}', error_mode: :strict2)
+    end
+  end
+
+  def test_decrement_strict2_accepts_valid_variable_name
+    template = Template.parse('{% decrement my-var %}', error_mode: :strict2)
+    assert_equal('-1', template.render)
+  end
+
+  def test_increment_strict2_rejects_empty_variable_name
+    assert_raises(Liquid::SyntaxError) do
+      Template.parse('{% increment %}', error_mode: :strict2)
+    end
+  end
+
+  def test_decrement_strict2_rejects_empty_variable_name
+    assert_raises(Liquid::SyntaxError) do
+      Template.parse('{% decrement %}', error_mode: :strict2)
+    end
+  end
 end

test/integration/tags/render_tag_test.rb

@@ -174,16 +174,16 @@ class RenderTagTest < Minitest::Test
   def test_increment_is_isolated_between_renders
     assert_template_result(
       '010',
-      '{% increment %}{% increment %}{% render "incr" %}',
-      partials: { 'incr' => '{% increment %}' },
+      '{% increment port %}{% increment port %}{% render "incr" %}',
+      partials: { 'incr' => '{% increment port %}' },
     )
   end
 
   def test_decrement_is_isolated_between_renders
     assert_template_result(
       '-1-2-1',
-      '{% decrement %}{% decrement %}{% render "decr" %}',
-      partials: { 'decr' => '{% decrement %}' },
+      '{% decrement port %}{% decrement port %}{% render "decr" %}',
+      partials: { 'decr' => '{% decrement port %}' },
     )
   end