Write vendored compact_index files atomically

The completeness check skips the download once every expected file exists,
but File.write truncates before writing, so an install interrupted partway
leaves a half-written file in place. A later run would see it, treat the
vendor tree as complete, and load a truncated source.

Download each file to a sibling .tmp path and rename it into place. The
rename stays within the vendor tree, so it is atomic and a target file is
only ever observed fully written.

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

Author: hsbt

spec/support/rubygems_ext.rb

@@ -110,7 +110,9 @@ def install_vendored_compact_index
           url = "https://raw.githubusercontent.com/rubygems/rubygems.org/#{ref}/#{path}"
           target = target_root.join(path)
           FileUtils.mkdir_p(File.dirname(target))
-          File.write(target, URI.parse(url).open(&:read))
+          tmp = "#{target}.tmp"
+          File.write(tmp, URI.parse(url).open(&:read))
+          File.rename(tmp, target)
         end
       end
     end