feat: implement database service with SQL Server and MySQL support

- Add database types (QueryResult, TransactionResult)
- Implement DatabaseService with connection management for SQL Server and MySQL
- Add query execution methods for both database types
- Include comprehensive error handling and logging
- Add unit test for error handling
- Configure Jest for testing

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tests/unit/services/database.service.test.ts

@@ -0,0 +1,26 @@
+import { DatabaseService } from '../../../src/main/services/database.service';
+
+describe('DatabaseService', () => {
+  it('should throw error when querying without connection', async () => {
+    const service = new DatabaseService(
+      {
+        server: 'localhost',
+        database: 'test',
+        username: 'test',
+        password: 'test',
+        driver: 'test',
+      },
+      {
+        host: 'localhost',
+        port: 3306,
+        database: 'test',
+        username: 'test',
+        password: 'test',
+      }
+    );
+
+    await expect(
+      service.executeSQLServerQuery('SELECT 1')
+    ).rejects.toThrow('SQL Server not connected');
+  });
+});