ci: parallelize RSpec and merge worker coverage (#7173)

[augustocbx]

What github issue is this PR for, if any?

Refs #7173 (Phase 1 only; Phases 2 and 3 remain open).

What changed, and why?

• Prepare four test databases and run the RSpec suite in four processes to reduce CI wall time.
• Write runtime data to separate worker logs, combine it after the suite, and cache it so later runs can balance groups by measured duration. A fresh run uses file size grouping until runtime data is available.
• Keep SimpleCov resultsets separate per worker and collate all four before the Qlty upload, preserving complete coverage reporting.
• Load spec_helper explicitly in parallel workers and add regression specs for the workflow and worker coverage paths.

How is this tested? (please write rspec and jest tests!) 💖💪

• RAILS_ENV=test PARALLEL_TEST_PROCESSORS=4 bundle exec rake parallel:create parallel:load_schema — passed.
• RAILS_ENV=test TEST_MAX_DURATION=60 RUN_SIMPLECOV=true bundle exec parallel_test spec --type rspec -n 4 --group-by filesize — 3,744 examples, 0 failures, 21 pending.
• bundle exec rspec spec/config — 25 examples, 0 failures.
• Combined the four worker runtime logs and verified runtime grouping (114, 116, 117, and 117 spec files). Collated all four coverage resultsets into coverage/.resultset.json.
• bin/lint and actionlint .github/workflows/rspec.yml — passed.

Screenshots please :)

Not applicable; this changes CI configuration and coverage reporting only.

Follow-up work

Phases 2 and 3 remain tracked in #7173.