🏗️(y-provider) organize yjs server

Many routes were in the server.ts file, now they
are in their own files in the handlers folder.
The server.ts file is now AppServer that handles
the routes.
We split as well the tests.

src/frontend/servers/y-provider/__tests__/convertMarkdown.test.ts

@@ -0,0 +1,66 @@
+import request from 'supertest';
+
+const port = 5556;
+const origin = 'http://localhost:3000';
+
+jest.mock('../src/env', () => {
+  return {
+    PORT: port,
+    COLLABORATION_SERVER_ORIGIN: origin,
+    Y_PROVIDER_API_KEY: 'yprovider-api-key',
+  };
+});
+
+import { initServer } from '../src/servers/appServer';
+
+console.error = jest.fn();
+const { app, server } = initServer();
+
+describe('Server Tests', () => {
+  afterAll(() => {
+    server.close();
+  });
+
+  test('POST /api/convert-markdown with incorrect API key should return 403', async () => {
+    const response = await request(app as any)
+      .post('/api/convert-markdown')
+      .set('Origin', origin)
+      .set('Authorization', 'wrong-api-key');
+
+    expect(response.status).toBe(403);
+    expect(response.body.error).toBe('Forbidden: Invalid API Key');
+  });
+
+  test('POST /api/convert-markdown with a Bearer token', async () => {
+    const response = await request(app as any)
+      .post('/api/convert-markdown')
+      .set('Origin', origin)
+      .set('Authorization', 'Bearer test-secret-api-key');
+
+    // Warning: Changing the authorization header to Bearer token format will break backend compatibility with this microservice.
+    expect(response.status).toBe(403);
+  });
+
+  test('POST /api/convert-markdown with missing body param content', async () => {
+    const response = await request(app as any)
+      .post('/api/convert-markdown')
+      .set('Origin', origin)
+      .set('Authorization', 'yprovider-api-key');
+
+    expect(response.status).toBe(400);
+    expect(response.body.error).toBe('Invalid request: missing content');
+  });
+
+  test('POST /api/convert-markdown with body param content being an empty string', async () => {
+    const response = await request(app as any)
+      .post('/api/convert-markdown')
+      .set('Origin', origin)
+      .set('Authorization', 'yprovider-api-key')
+      .send({
+        content: '',
+      });
+
+    expect(response.status).toBe(400);
+    expect(response.body.error).toBe('Invalid request: missing content');
+  });
+});