Add rb_len_t to RbConfig::SIZEOF

An extconf.rb can then tell whether `rb_len_t` exists and how wide it is without compiling a probe.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ext/rbconfig/sizeof/extconf.rb

@@ -32,5 +32,6 @@ have_type('_Decimal128') # x86_64 ABI
 have_type('__m64') # x86_64 ABI (optional)
 have_type('__m128') # x86_64 ABI (optional)
 have_type('__float80') # gcc x86
+have_type('rb_len_t')
 
 create_makefile('rbconfig/sizeof')

test/test_rbconfig.rb

@@ -52,6 +52,11 @@ class TestRbConfig < Test::Unit::TestCase
     end
   end
 
+  def test_sizeof_rb_len_t
+    require 'rbconfig/sizeof'
+    assert_include([RbConfig::SIZEOF["long"], RbConfig::SIZEOF["void*"]], RbConfig::SIZEOF["rb_len_t"])
+  end
+
   def test_limits_and_sizeof_access_in_ractor
     assert_separately(["-W0"], <<~'RUBY')
       r = Ractor.new do