Refactor Pathname.mktmpdir helpers to class methods

Unify mktmpdir helper methods as private class methods instead of
mixing class and instance methods. Inline random_name into make_tmpdir
to remove the unnecessary instance method.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

pathname.rb

@@ -1433,8 +1433,7 @@ def rmtree
   #   end
   #
   def self.mktmpdir
-    tmpdir = self.new(resolve_tmpdir)
-    path = tmpdir.__send__(:make_tmpdir)
+    path = make_tmpdir(resolve_tmpdir)
     if block_given?
       begin
         yield path
@@ -1475,24 +1474,22 @@ def self.resolve_tmpdir
     return systmpdir if File.directory?(systmpdir) && File.writable?(systmpdir)
     '/tmp'
   end
+  private_class_method :resolve_tmpdir
 
-  def make_tmpdir
+  def self.make_tmpdir(tmpdir)
     t = Time.now.strftime("%Y%m%d")
     n = nil
     begin
-      name = "d#{t}-#{$$}-#{random_name}#{n ? "-#{n}" : ''}"
-      path = File.join(@path, name)
+      name = "d#{t}-#{$$}-#{Random.urandom(4).unpack1("L").%(36**6).to_s(36)}#{n ? "-#{n}" : ''}"
+      path = File.join(tmpdir, name)
       Dir.mkdir(path, 0700)
     rescue Errno::EEXIST
       n = (n || 0) + 1
       retry
     end
-    self.class.new(path)
-  end
-
-  def random_name
-    Random.urandom(4).unpack1("L").%(36**6).to_s(36)
+    new(path)
   end
+  private_class_method :make_tmpdir
 end
 
 class Pathname