ci: use default bundler on ruby head

[rdavid]

The experimental ruby-head job fails with
Bundler::CorruptBundlerInstallError: setup-ruby installs Bundler 2.7.1
(from the Gemfile.lock BUNDLED WITH section), which clashes with the
4.1.0.dev Bundler specification shipped as a default gem in ruby-head
snapshots.

Sets bundler: default for the head matrix entry only, so the job uses
the Bundler bundled with the snapshot. Stable rubies keep the
Gemfile.lock behavior.

Lint results: yamllint --strict, actionlint, and zizmor . all pass.

Summary by CodeRabbit

• Chores
  • Updated automated Ruby workflow configuration to support flexible Bundler settings.
  • Standardized dependency installation to use the lockfile by default, with an override for specific Ruby environments.

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 7f3f52ab-92ef-4dd3-99ab-0454ee25abd5

📥 Commits

Reviewing files that changed from the base of the PR and between 99680cb and 6cac805.

📒 Files selected for processing (1)

• .github/workflows/rake.yml

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📝 Walkthrough

Walkthrough

The rake workflow now supports per-entry Bundler configuration, using Gemfile.lock by default and selecting the default Bundler behavior for the Ruby head matrix entry.

Changes

Rake workflow

Layer / File(s)	Summary
Matrix-specific Bundler selection .github/workflows/rake.yml	The Ruby head matrix entry sets bundler: default, while the setup step uses the matrix value or falls back to Gemfile.lock.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Suggested reviewers: dependabot[bot]

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly reflects the CI change for ruby head using the default Bundler instead of the lockfile setting.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-ruby-head-bundler

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