[monitor, theia] monitor service incorrectly responds with 200 if the request body is malformed

### Describe the bug

The response ends earlier before setting the 400 status code:

https://github.com/eclipse-theia/theia-cloud/blob/55813e2c212b0b787fa4181affe466f987f0337a/theia/extensions/monitor-theia/src/node/modules/messaging-backend-module-impl.ts#L22-L23

### Expected behavior

`POST /monitor/message` responds with 400 if the request body is invalid

### Cluster provider

_No response_

### Version

_No response_

### Additional information

_No response_

Start in theia/extensions/monitor-theia/src/node/modules/messaging-backend-module-impl.ts at the referenced lines, then reproduce a malformed POST to /monitor/message. Done means the endpoint responds with HTTP 400 for an invalid request body instead of 200.

Written by the indexing model from the issue text.