Coerce the post-install message to a String before sanitizing

post_install_message may be a non-String such as an array, so call to_s
before clean_text to avoid raising during install.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hsbt

lib/rubygems/installer.rb

@@ -299,7 +299,7 @@ def install
 
     File.chmod(dir_mode, gem_dir) if dir_mode
 
-    say clean_text(spec.post_install_message) if options[:post_install_message] && !spec.post_install_message.nil?
+    say clean_text(spec.post_install_message.to_s) if options[:post_install_message] && !spec.post_install_message.nil?
 
     Gem::Specification.add_spec(spec) unless @install_dir
 

test/rubygems/test_gem_installer.rb

@@ -1498,6 +1498,22 @@ def test_install_sanitizes_post_install_message
     refute_match(/\e\]2;pwn/, @ui.output)
   end
 
+  def test_install_handles_non_string_post_install_message
+    # post_install_message may be a non-String (the gemspec schema allows an
+    # array), so sanitizing must not assume it responds to gsub.
+    @spec = setup_base_spec
+    @spec.post_install_message = %w[one two]
+
+    installer = Gem::Installer.for_spec @spec, post_install_message: true
+    installer.gem_home = @gemhome
+
+    use_ui @ui do
+      installer.install
+    end
+
+    assert_match(/one/, @ui.output)
+  end
+
   def test_install_extension_dir
     gemhome2 = "#{@gemhome}2"