Parallel Tests 5.7 Prepare Documentation

Context

The feature branch updates parallel_tests from 5.6 to 5.7. Version 5.7 changes parallel:prepare to stop dumping the schema before loading parallel test databases, matching Rails behavior.

Current documentation says developers can run parallel:prepare after migrations and describes it as loading the schema into each parallel database. That remains true, but the documentation does not state that the schema file must already be current.

Design

Keep the 5.7 dependency and upstream rake-task behavior. Update the parallel RSpec setup guide to state that:

• the normal migration or Easy8 installation workflow must update db/schema.rb first;
• parallel:prepare loads the existing schema into every parallel test database;
• parallel:prepare no longer runs db:schema:dump itself.

Keep the documented reset sequence (parallel:drop, parallel:create, parallel:prepare) because it remains valid when db/schema.rb is current.

Do not override parallel:prepare or replace it with parallel:migrate. An override would diverge from upstream 5.7 behavior, while parallel migrations would be slower and change the documented setup model.

Scope

Change only docs/backend_tutorials/parallel-rspec-setup.md. No application code, rake tasks, database migrations, or dependency constraints beyond the existing 5.7 update are needed.

Verification

• Confirm documentation no longer implies that parallel:prepare creates or refreshes db/schema.rb.
• Confirm commands for preparing and resetting parallel databases remain accurate for parallel_tests 5.7.
• Review the final diff for documentation-only scope.