Add Postgres + MySQL CI matrix (#57)

Adds a test_db job covering rails_8.1 against postgresql and mysql2
with service containers, plus the infrastructure to make the dummy
app's adapter configurable via FIXTURE_KIT_DB.

- Bundler groups for pg and mysql2; appraisal lockfiles regenerated
- spec/dummy/config/database.yml is ENV-driven (FIXTURE_KIT_DB and
  friends), defaults stay sqlite
- ensure_test_databases_exist! creates the PG/MySQL databases on
  first run via a separate connection class so the main
  ActiveRecord::Base connection is established cleanly later
- drop_table calls use force: :cascade to handle PG's FK dependencies
- spec/dummy gets its own bundle install in CI with BUNDLE_GEMFILE
  unset (the parent matrix's gemfile path doesn't resolve from inside
  spec/dummy)
- ActiveRecordCoder coverage tests the upsert_all path via a helper
  that omits :unique_by on MySQL/Trilogy (those adapters reject it)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>