BUG: Existing draft application validation is logged as unhandled

**Describe the Bug**

Creating a draft when the student already has one returns 422 and is emitted as an unhandled exception.

**Expected Behaviour**

The existing-draft condition is handled as an expected client response with useful navigation or feedback and without a misleading unhandled-exception error log.

**Actual Behaviour**

`createDraftApplication` throws `UnprocessableEntityException`, which `AppAllExceptionsFilter` records as unhandled.

**Related Issue**

#3557 tracks improving the user flow when an application is already in progress. This report captures the production API logging behavior.

**Log**

```text
[Nest] 18 - 09/04/2026, 7:06:24 AM LOG [AccessLoggerMiddleware] Request - POST /api/students/application/draft From [redacted] | User GUID: [redacted] | User Agent: Firefox 148 on macOS
[Nest] 18 - 09/04/2026, 7:06:24 AM ERROR [AppAllExceptionsFilter] Unhandled exception, request path: /api/students/application/draft
{
"stack": "UnprocessableEntityException: There is already an existing draft application associated with the current student.\n at ApplicationStudentsController.createDraftApplication (webpack://studentaid-backend/apps/api/src/route-controllers/application/application.students.controller.ts:474:15)",
"response": { "message": "There is already an existing draft application associated with the current student." },
"status": 422,
"message": "There is already an existing draft application associated with the current student.",
"name": "UnprocessableEntityException"
}
```

Start in apps/api/src/route-controllers/application/application.students.controller.ts around createDraftApplication and trace how AppAllExceptionsFilter handles the resulting 422. Reproduce the existing-draft request and verify the client response remains useful while the expected condition is no longer recorded as an unhandled exception.

Written by the indexing model from the issue text.