Add missing codecov gem to unblock SimpleCov coverage reporting

[actowery]

Summary

spec_helper.rb's SIMPLECOV=yes gate (via puppetlabs_spec_helper's module_spec_helper) requires simplecov, simplecov-console, and codecov to all be loadable. simplecov-console was already declared in the Gemfile and simplecov was present only transitively, but codecov was missing entirely. This PR adds gem "codecov", '~> 0.6', require: false to the Gemfile's :development group — no other changes.

Additional Context

• Root cause and the steps to reproduce: running SIMPLECOV=yes bundle exec rake spec on main raises LoadError: cannot load such file -- codecov at spec_helper.rb:7, which puppetlabs_spec_helper wraps into a generic "add the simplecov, simplecov-console, codecov gems to Gemfile" error. Surfaced during a DevX coverage audit across PDK/Bolt-adjacent tooling (PPM-1579).
• Thought process: codecov is required by the existing gate but was never added as a dependency — this just supplies the missing piece rather than changing any gating behavior.

Related Issues (if any)

Surfaced as part of PPM-1579 (Quality Improvements for H2 2026) coverage audit — no existing issue filed against this repo specifically.

Checklist

• 🟢 Spec tests. — SIMPLECOV=yes bundle exec rake spec now completes and reports 82.02% line coverage (406/495) instead of erroring. Note: 3 pre-existing failures in spec/integration/puppetcore_spec.rb (missing PUPPET_FORGE_TOKEN env var) are unrelated to this change.
• 🟢 Acceptance tests. — not run (this is a dev/coverage-tooling-only dependency change, no acceptance-test surface affected).
• Manually verified. — verified locally under Ruby 3.3.11; note CI runs Ruby 3.1 (ci.yml), not independently verified against 3.1 but this is a pure Gemfile dependency addition with no version-specific code.