[Bug]: UI drops selected user from payload when creating LDAP User Connection

### Describe the bug

When attempting to manually create an LDAP User Connection via the Authentik Web UI, the frontend drops the selected "User" field from the JSON payload when submitting the form to the API. This results in the API receiving a `null` user value, causing the creation to fail.

### How to reproduce

1. Go to the Authentik Admin UI.
2. Navigate to create a new LDAP User Connection (`/api/v3/sources/user_connections/ldap/`).
3. Select a valid target user from the dropdown.
4. Select the source and fill in the required identifier (e.g., `S-1-5-21-...`).
5. Submit the form.
6. Inspect the network request and observe the missing `user` field in the POST payload.

### Expected behavior

The UI should correctly include the selected `user` ID in the POST payload sent to the API.

### Screenshots

### Additional context

This issue is linked to a backend validation bug where the API does not catch the missing field and instead returns a 500 error.

### Deployment Method

Docker

### Version

2026.8.1

### Relevant log output

```shell

```

Start at the Authentik Admin UI form for /api/v3/sources/user_connections/ldap/ and inspect the POST payload after selecting a User. Confirm the selected user ID is preserved, then exercise the form flow and verify the request includes user; it is done when manual creation no longer omits that field.

Written by the indexing model from the issue text.