Extend all test tasks to run against every configured test database

When database.yml defines more than one top-level key starting with "test",
every standard test task (rails test, rails test:models, rails test:system,
rails test:controllers, …) is automatically rewritten at load time to run
the full suite against each configured database in turn.

Single-database setups are completely unaffected — the wrapping code
activates only when test_configs.size > 1, so existing behaviour is
preserved by default.

Convention: test: is the required primary; test_<name>: adds an adapter:
  test_sqlite:
    adapter: sqlite3
    database: db/fixinme_test.sqlite3
  test_pg:
    adapter: postgresql
    ...

Mechanism:
- lib/tasks/test_databases.rake loads after railties/testing.rake (which
  defines all test tasks). For each task in the wrapped set it calls
  Rake::Task[name].clear_actions and re-enhances with a block that loops
  over DB configs, writing a temporary database.yml per database and
  running "rails <task_name>" as a subprocess with RAILS_DATABASE_YML set.
- config/application.rb(.dist): reads RAILS_DATABASE_YML and overrides
  Rails' database config path before initialisation — no monkey-patching
  of the test runner required.
- Adapter gem availability is checked before each run; missing adapters are
  skipped with a hint on which bundle group to enable.
- A formatted summary (✓/✗ per database) is printed; exits non-zero on
  any failure.

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

config/database.yml.dist

@@ -48,3 +48,24 @@ production:
 #test:
 #  <<: *default
 #  database: fixinme_test
+
+# Multi-database testing
+# ----------------------
+# Any key starting with "test" is treated as a test database.
+# When more than one is present, EVERY test task (rails test, rails test:models,
+# rails test:system, …) automatically runs against all of them.
+#
+# The adapter gem must be available:
+#   bundle config --local with "mysql sqlite"   # mysql + sqlite
+#   bundle config --local with "mysql pg"       # mysql + postgresql
+#
+#test_sqlite:
+#  adapter: sqlite3
+#  database: db/fixinme_test.sqlite3
+#
+#test_pg:
+#  adapter: postgresql
+#  database: fixinme_test
+#  username: fixinme
+#  password: Some-password1%
+#  host: localhost