Skip bundler self-checksum on ruby-core in test fixtures (#9506)

* Skip bundler self-checksum on ruby-core in test fixtures

In ruby-core test setup, bundler is loaded as a default gem and the
bundler.gem cache file is not present on disk where
LockfileGenerator#bundler_checksum looks for it. As a result, the
generator omits the bundler checksum from the regenerated lockfile,
while the test's checksums_section helper still adds it, making the
"does not change the lock" expectations in setup_spec.rb fail on
ruby-core CI for release branches (where Bundler::VERSION does not end
in .dev). Mirror the generator's give-up conditions in the helper so
that test fixtures and the regenerated lockfile stay in sync.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: 3 people

spec/support/checksums.rb

@@ -58,7 +58,12 @@ def checksums_section(enabled = true, bundler_checksum: true, &block)
       ChecksumsBuilder.new(enabled, &block).tap do |builder|
         next if builder.bundler_registered || !bundler_checksum
 
-        next if Bundler::VERSION.to_s.end_with?(".dev")
+        # Mirror the conditions under which LockfileGenerator#bundler_checksum
+        # gives up and omits the bundler checksum from the lockfile:
+        #   - .dev: development builds have no released bundler.gem to checksum.
+        #   - ruby_core?: bundler is loaded as a default gem, so bundler.gem
+        #     is not present on disk under the test gem cache.
+        next if Bundler::VERSION.to_s.end_with?(".dev") || Spec::Path.ruby_core?
         builder.checksum(system_gem_path, "bundler", Bundler::VERSION, Gem::Platform::RUBY, "cache")
       end
     end