[Bug]: Missing API validation for user field on LDAP User Connection creation causes 500 Error

### Describe the bug

The `/api/v3/sources/user_connections/ldap/` endpoint lacks serializer validation for the required `user` field. If a POST request is sent without this field (or if the UI drops it), the API passes `null` to the database instead of catching the missing field. This results in a PostgreSQL `IntegrityError` (violating the `not-null` constraint) and a hard `500 Internal Server Error` rather than a graceful `400 Bad Request`.

### How to reproduce

1. Send a POST request to `/api/v3/sources/user_connections/ldap/` omitting the `user` field (or use the Authentik UI which currently suffers from a bug that drops this field).
2. Observe the `500 Internal Server Error` in the API response and server logs.

### Expected behavior

The API serializer should validate that `user` is provided in the payload. If missing, it should return a `400 Bad Request` explicitly stating that the user field is required, preventing a database constraint violation.

### Screenshots

_No response_

### Additional context

This was discovered because the Authentik UI currently fails to pass the selected user when submitting the form, immediately triggering this backend crash.

### Deployment Method

Docker

### Version

2026.8.1

### Relevant log output

```shell
// PostgreSQL Log:
postgresql-1 | ERROR: null value in column "user_id" of relation "authentik_core_usersourceconnection" violates not-null constraint
postgresql-1 | DETAIL: Failing row contains (587, 2026-09-02 07:10:11.116352+00, 2026-09-02 07:10:11.116374+00, f220ba0b-87b9-4bfb-8d35-642956a105cd, null, S-1-5-21-3638675543-1746918860-1899855899-1109).

// Authentik Server Log:
{"event": "Internal Server Error: /api/v3/sources/user_connections/ldap/", "exception": [{"exc_type": "IntegrityError", "exc_value": "null value in column \"user_id\" of relation \"authentik_core_usersourceconnection\" violates not-null constraint..."]}]}
```

Start at the serializer handling POST requests for /api/v3/sources/user_connections/ldap/ and trace how a missing user field reaches PostgreSQL. Reproduce the request without user and verify that validation returns a 400 Bad Request identifying user as required, without an IntegrityError or 500 response.

Written by the indexing model from the issue text.