feat: Add reusable workflow for extension testing

Add a reusable workflow that handles database services (PostgreSQL, MySQL)
so extensions don't need to define them manually. The workflow accepts
individual inputs for rails_versions, ruby_versions, solidus_branches,
and databases with sensible defaults.

This provides a simpler alternative to the composite action while
maintaining backwards compatibility.

Author: tvdeyen

.github/workflows/test.yml

@@ -0,0 +1,67 @@
+name: Test Solidus Extension
+
+on:
+  workflow_call:
+    inputs:
+      rails_versions:
+        description: 'Rails versions to test (JSON array)'
+        type: string
+        default: '["8.0", "7.2"]'
+      ruby_versions:
+        description: 'Ruby versions to test (JSON array)'
+        type: string
+        default: '["3.4"]'
+      solidus_branches:
+        description: 'Solidus branches to test (JSON array)'
+        type: string
+        default: '["v4.6", "v4.5"]'
+      databases:
+        description: 'Databases to test (JSON array): postgresql, mysql, sqlite'
+        type: string
+        default: '["postgresql", "mysql", "sqlite"]'
+
+jobs:
+  test:
+    runs-on: ubuntu-24.04
+    strategy:
+      fail-fast: false
+      matrix:
+        rails_version: ${{ fromJSON(inputs.rails_versions) }}
+        ruby_version: ${{ fromJSON(inputs.ruby_versions) }}
+        solidus_branch: ${{ fromJSON(inputs.solidus_branches) }}
+        database: ${{ fromJSON(inputs.databases) }}
+
+    services:
+      postgres:
+        image: postgres:16
+        env:
+          POSTGRES_HOST_AUTH_METHOD: trust
+        options: >-
+          --health-cmd="pg_isready"
+          --health-interval=10s
+          --health-timeout=5s
+          --health-retries=5
+        ports:
+          - 5432:5432
+      mysql:
+        image: mysql:8
+        env:
+          MYSQL_ALLOW_EMPTY_PASSWORD: "yes"
+        options: >-
+          --health-cmd="mysqladmin ping"
+          --health-interval=10s
+          --health-timeout=5s
+          --health-retries=5
+        ports:
+          - 3306:3306
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Run extension tests
+        uses: solidusio/test-solidus-extension@main
+        with:
+          database: ${{ matrix.database }}
+          rails-version: ${{ matrix.rails_version }}
+          ruby-version: ${{ matrix.ruby_version }}
+          solidus-branch: ${{ matrix.solidus_branch }}

README.md

@@ -1,8 +1,80 @@
 # Test Solidus Extension GitHub action
 
-A GitHub action for testing Solidus extensions
+GitHub actions and workflows for testing Solidus extensions.
 
-## Example configuration
+## Usage
+
+### Reusable Workflow (Recommended)
+
+The simplest way to test your extension. The reusable workflow handles database services and provides a sensible default test matrix:
+
+```yaml
+name: Test
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  test:
+    uses: solidusio/test-solidus-extension/.github/workflows/test.yml@main
+```
+
+#### Inputs
+
+| Input | Description | Default |
+|-------|-------------|---------|
+| `rails_versions` | Rails versions (JSON array) | `["8.0", "7.2"]` |
+| `ruby_versions` | Ruby versions (JSON array) | `["3.4"]` |
+| `solidus_branches` | Solidus branches (JSON array) | `["v4.6", "v4.5"]` |
+| `databases` | Databases (JSON array) | `["postgresql", "mysql", "sqlite"]` |
+
+#### Custom Matrix
+
+Override any input to customize the test matrix:
+
+```yaml
+jobs:
+  test:
+    uses: solidusio/test-solidus-extension/.github/workflows/test.yml@main
+    with:
+      rails_versions: '["7.2"]'
+      databases: '["postgresql", "sqlite"]'
+```
+
+### Composite Action
+
+For full control over your workflow, use the composite action directly. This requires you to define database services yourself.
+
+#### Inputs
+
+| Input | Description | Required | Default |
+|-------|-------------|----------|---------|
+| `ruby-version` | Ruby version to use | Yes | `3.3` |
+| `rails-version` | Rails version to use | Yes | `7.2` |
+| `solidus-branch` | Solidus branch to use | Yes | `main` |
+| `database` | Database to use (`sqlite`, `postgresql`, `mysql`, or `mariadb`) | Yes | `sqlite` |
+
+#### Database Services
+
+The composite GitHub Action **cannot define services directly**. If you need to test against PostgreSQL or MySQL/MariaDB, you must define the database service in your workflow file.
+
+The action will install the necessary database client libraries automatically based on the `database` input.
+
+#### Database Credentials
+
+The action automatically sets the correct `DB_USERNAME` based on the database:
+
+- **PostgreSQL**: `postgres` (default superuser)
+- **MySQL/MariaDB**: `root` (default superuser)
+
+Configure your database services with passwordless access for simplicity (see example below).
+
+> [!WARNING]
+> Rails 8.0 has a [known issue](https://github.com/rails/rails/issues/53673) with MySQL/MariaDB that causes empty `Mysql2::Error` messages. Until a fix is released, exclude the `mariadb` + Rails 8.0 combination from your test matrix.
+
+#### Example Configuration
 
 ```yaml
 name: Test