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]>

lib/liquid/tags/decrement.rb

@@ -23,13 +23,29 @@ module Liquid
   #   {% decrement variable_name %}
   # @liquid_syntax_keyword variable_name The name of the variable being decremented.
   class Decrement < Tag
+    include ParserSwitching
+
     attr_reader :variable_name
 
     def initialize(tag_name, markup, options)
       super
+      parse_with_selected_parser(markup)
+    end
+
+    def lax_parse(markup)
       @variable_name = markup.strip
     end
 
+    def strict_parse(markup)
+      lax_parse(markup)
+    end
+
+    def strict2_parse(markup)
+      p = @parse_context.new_parser(markup.strip)
+      @variable_name = p.consume(:id)
+      p.consume(:end_of_string)
+    end
+
     def render_to_output_buffer(context, output)
       counter_environment = context.environments.first
       value = counter_environment[@variable_name] || 0

lib/liquid/tags/increment.rb

@@ -23,13 +23,29 @@ module Liquid
   #   {% increment variable_name %}
   # @liquid_syntax_keyword variable_name The name of the variable being incremented.
   class Increment < Tag
+    include ParserSwitching
+
     attr_reader :variable_name
 
     def initialize(tag_name, markup, options)
       super
+      parse_with_selected_parser(markup)
+    end
+
+    def lax_parse(markup)
       @variable_name = markup.strip
     end
 
+    def strict_parse(markup)
+      lax_parse(markup)
+    end
+
+    def strict2_parse(markup)
+      p = @parse_context.new_parser(markup.strip)
+      @variable_name = p.consume(:id)
+      p.consume(:end_of_string)
+    end
+
     def render_to_output_buffer(context, output)
       counter_environment = context.environments.first
       value = counter_environment[@variable_name] || 0