Add multi-adapter test support: SQLite + MySQL via Gitea Actions and rake task

- .gitea/workflows/test.yml: two parallel CI jobs (SQLite and MySQL),
  each generates its own database.yml inline and runs the test suite
- lib/tasks/test_multi_db.rake: `rails test:all_adapters` runs both
  adapters sequentially using DATABASE_URL to switch at runtime

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

.gitea/workflows/test.yml

@@ -0,0 +1,95 @@
+name: Tests
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  test-sqlite:
+    name: Tests (SQLite)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.3'
+          bundler-cache: true
+        env:
+          BUNDLE_WITH: "sqlite:development:test"
+
+      - name: Generate database.yml
+        run: |
+          cat > config/database.yml <<'EOF'
+          test:
+            adapter: sqlite3
+            database: db/test.sqlite3
+            pool: 5
+          EOF
+
+      - name: Set up test database
+        run: bin/rails db:create db:schema:load
+        env:
+          RAILS_ENV: test
+
+      - name: Run tests
+        run: bin/rails test
+        env:
+          RAILS_ENV: test
+          CI: "true"
+
+  test-mysql:
+    name: Tests (MySQL)
+    runs-on: ubuntu-latest
+
+    services:
+      mysql:
+        image: mysql:8.0
+        env:
+          MYSQL_ROOT_PASSWORD: ""
+          MYSQL_ALLOW_EMPTY_PASSWORD: "yes"
+          MYSQL_DATABASE: fixin_test
+        ports:
+          - 3306:3306
+        options: >-
+          --health-cmd="mysqladmin ping"
+          --health-interval=10s
+          --health-timeout=5s
+          --health-retries=3
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.3'
+          bundler-cache: true
+        env:
+          BUNDLE_WITH: "mysql:development:test"
+
+      - name: Generate database.yml
+        run: |
+          cat > config/database.yml <<'EOF'
+          test:
+            adapter: mysql2
+            encoding: utf8mb4
+            collation: utf8mb4_0900_as_ci
+            database: fixin_test
+            host: 127.0.0.1
+            username: root
+            password: ""
+            pool: 5
+          EOF
+
+      - name: Set up test database
+        run: bin/rails db:schema:load
+        env:
+          RAILS_ENV: test
+
+      - name: Run tests
+        run: bin/rails test
+        env:
+          RAILS_ENV: test
+          CI: "true"