BUG: Supporting-user self-association validation is logged as unhandled

**Describe the Bug**

A supporting user attempting to provide data for an application associated with the same identity receives 422 and the validation is emitted as an unhandled exception.

**Expected Behaviour**

The self-association validation is handled as an expected client response with appropriate feedback and without a misleading unhandled-exception error log.

**Actual Behaviour**

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

**Log**

```text
[Nest] 18 - 09/04/2026, 7:22:31 AM LOG [AccessLoggerMiddleware] Request - POST /api/supporting-users/supporting-user/application From [redacted] | User GUID: [redacted] | User Agent: Firefox 154 on macOS
[Nest] 18 - 09/04/2026, 7:22:31 AM ERROR [AppAllExceptionsFilter] Unhandled exception, request path: /api/supporting-users/supporting-user/application
{
"stack": "UnprocessableEntityException: The user searching for applications to provide data must be different from the user associated with the student application.\n at SupportingUserSupportingUsersController.getApplicationDetails (webpack://studentaid-backend/apps/api/src/route-controllers/supporting-user/supporting-user.supporting-users.controller.ts:104:13)",
"status": 422,
"message": "The user searching for applications to provide data must be different from the user associated with the student application.",
"name": "UnprocessableEntityException"
}
```

Start in apps/api/src/route-controllers/supporting-user/supporting-user.supporting-users.controller.ts at SupportingUserSupportingUsersController.getApplicationDetails, then inspect AppAllExceptionsFilter and its handling of UnprocessableEntityException. Reproduce the self-association request and verify the 422 still provides appropriate feedback without being logged as an unhandled exception.

Written by the indexing model from the issue text.