💩(y-provider) init a markdown converter endpoint

This code is quite poor. Sorry, I don't have much time working
on this feature. However, it should be functional.

I've reused the code we created for the Demo with Kasbarian.
I've not tested it yet with all corner case. Error handling
might be improved for sure, same for logging.

This endpoint is not modular. We could easily introduce options
to modify its behavior based on some options. YAGNI

I've added bearer token authentification, because it's unclear
how this micro service would be exposed. It's totally not required
if the microservice is not exposed through an Ingress.

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

@@ -91,6 +91,39 @@ describe('Server Tests', () => {
     hocuspocusServer.closeConnections = closeConnections;
   });
 
+  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 missing body param content', async () => {
+    const response = await request(app as any)
+      .post('/api/convert-markdown')
+      .set('Origin', origin)
+      .set('Authorization', 'test-secret-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', 'test-secret-api-key')
+      .send({
+        content: '',
+      });
+
+    expect(response.status).toBe(400);
+    expect(response.body.error).toBe('Invalid request: missing content');
+  });
+
   ['/collaboration/api/anything/', '/', '/anything'].forEach((path) => {
     test(`"${path}" endpoint should be forbidden`, async () => {
       const response = await request(app as any).post(path);