Add multi-database test runner (test:all_databases)

Adds `bundle exec rails test:all_databases` which runs the full test
suite against every test database configured in database.yml in a single
command.

Convention: any top-level key starting with "test" that contains a Hash
is a test database. `test:` is the required primary; `test_<name>:` blocks
are optional additional adapters (e.g. test_sqlite, test_pg).

For each configured database the task:
  1. Checks the required adapter gem is available (skips with warning if not)
  2. Runs `rails db:test:prepare` to create and migrate the database
  3. Runs `rails test` and records pass/fail
  4. Prints a summary and exits non-zero if any database failed

Mechanism: a RAILS_DATABASE_YML env var points each subprocess to a
temporary database.yml that contains only the current test config.
config/application.rb(.dist) reads this var and overrides Rails'
database config path before initialisation, so no monkey-patching of
the test runner is required.

config/database.yml.dist is updated with documented examples for SQLite
and PostgreSQL additional test databases.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/application.rb.dist

@@ -20,6 +20,10 @@ Bundler.require(*Rails.groups)
 
 module FixinMe
   class Application < Rails::Application
+    # Allow RAILS_DATABASE_YML to override the database config file path.
+    # Used by `rails test:all_databases` to test against multiple DB adapters.
+    config.paths['config/database'] = [ENV['RAILS_DATABASE_YML']] if ENV['RAILS_DATABASE_YML']
+
     # Initialize configuration defaults for originally generated Rails version.
     config.load_defaults 7.0