`POST /_synapse/mas/delete_user` always triggers deactivations as self-deactivations, despite MAS Admin API being used

As at v1.151.0

`MasDeleteUserResource` has:
```python
await self.deactivate_account_handler.deactivate_account(
user_id=user_id.to_string(),
erase_data=body.erase,
requester=create_requester(user_id=user_id), # <---
)
```

The marked line makes the deactivation trigger as a self-deactivation, even though it could have been requested by an administrator in MAS.

We should also set `by_admin` properly.

Start at the MasDeleteUserResource implementation for POST /_synapse/mas/delete_user and trace the deactivate_account call, especially create_requester and the by_admin value. Confirm how administrator versus self-deactivation requests are represented, then update the request metadata so MAS administrator actions are marked by_admin and verify the resulting deactivation behavior.

Written by the indexing model from the issue text.