(CAT-2635) Add rubocop-hash_inspect to PDK rubocop gem group and plugins

[david22swan]

Summary

Ruby 3.4 (December 2024, Bug #20433) changed the output of Hash#inspect from the legacy rocket syntax to colon syntax:

Ruby version	{a: 1, "b" => 2}.inspect output
Ruby <= 3.3 (Puppet 8)	{:a=>1, "b"=>2}
Ruby >= 3.4 (Puppet 9 / Ruby 4)	{a: 1, "b" => 2}

Test assertions that hardcode the old format — for example eq("{:a=>1}") or match(/\{:name=>"foo"\}/) — silently break on Puppet 9. The rubocop-hash_inspect gem provides a single conservative cop (HashInspect/LegacyHashInspectFormat) that flags those literals statically during pdk validate, so module authors can fix them before upgrading.

This PR adds rubocop-hash_inspect ~> 0.2 to the :development gem group in config_defaults.yml (alongside rubocop-performance and rubocop-rspec) and adds 'rubocop-hash_inspect' to the defaults['plugins'] array in moduleroot/.rubocop.yml.erb, so the cop runs automatically on every pdk validate after a pdk update.

Additional Context

Before (Ruby 3.4 / Puppet 9 breaks silently):

expect(result.inspect).to eq("{:a=>1}")
expect(msg).to match(/\{:name=>"foo"\}/)

After — the cop flags these on pdk validate:

Offense: Legacy `Hash#inspect` format (`{:sym=>...}`). Ruby 3.4+ renders hashes as
`{sym: ...}`, so this hardcoded value breaks on Ruby 3.4 / Puppet 9. Update it to
the new format.

Correct form:

expect(result.inspect).to eq("{a: 1}")
expect(msg).to match(/\{name: "foo"\}/)
# Or better — use a matcher that doesn't depend on inspect output:
expect(result).to eq({ a: 1 })

Version constraint: ~> 0.2 targets the published 0.2.0 release (pessimistic minor lock — compatible with any 0.2.x patch but will not automatically pull in a future 0.3.0/1.0.0 that may change detection semantics).

On by default: The cop ships enabled at convention severity, matching rubocop-performance. It is non-blocking (PDK does not fail validate on convention offenses) and produces no false positives against the puppetlabs-stdlib and puppetlabs-concat baseline corpora.

A note on the ERB template comment: moduleroot/.rubocop.yml.erb carries the comment "note that these hashes need to use rockets, not colons, because rubocop config requires strings as keys". That refers to the ERB hash-key syntax used to render the template (writing 'plugins' => [...] rather than plugins: [...]), and is unrelated to the string values the cop flags. Config files are outside the cop's detection scope — only string and regexp literals in module source code are checked.

Related Issues (if any)

• Jira: CAT-2635 (parent epic CAT-2630 — PDK | Puppet 9 Compatibility Testing)
• Gem: rubocop-hash_inspect on RubyGems (0.2.0 live)
• Ruby Bug: #20433

Checklist

• 🟢 Spec tests.
• Manually verified.