Update infrastructure that handles parsing switching:

* Remove development helpers from parse context
* Simplify strict_parse_with_error_mode_fallback and update
  documentation
* Add unit tests for `Liquid::Expression` and `Liquid::ParseContext`
* Update test helpers to work better with the `:rigid` mode

lib/liquid/parse_context.rb

@@ -55,10 +55,15 @@ module Liquid
     end
 
     def parse_expression(markup, safe: false)
-      # todo(guilherme): remove this once rigid mode is fully using safe_parse_expression
       if !safe && @error_mode == :rigid
-        # raise Liquid::InternalError, "parse_expression is not supported in rigid mode"
-        puts("🚨 parse_expression used in rigid mode")
+        # parse_expression is a widely used API. To maintain backward
+        # compatibility while raising awareness about rigid parser standards,
+        # the safe flag supports API users make a deliberate decision.
+        #
+        # In rigid mode, markup MUST come from a string returned by the parser
+        # (e.g., parser.expression). We're not calling the parser here to
+        # prevent redundant parser overhead.
+        raise Liquid::InternalError, "unsafe parse_expression cannot be used in rigid mode"
       end
 
       Expression.parse(markup, @string_scanner, @expression_cache)