String#+ with a non-String raises NoMethodError instead of TypeError #409

Found while writing the #391 fixtures (verified 2026-09-27 on 64326cb against CRuby 3.4.8).

begin; "s" + 1; rescue => e; puts e.class; end

It looks like String#+ tries to_str on its argument and lets the resulting NoMethodError escape, where it should convert that into TypeError. The same coercion path (rb_str_to_str / implicit to_str) is probably shared by other String methods (<<, concat, start_with?, ...) and should be checked with them. method_missing_lite_serve.rb uses nil + 1 for its deopt case instead of "s" + 1 because of this.