Scope mappings not executed for proxy providers in 2026.2.4

### Describe the bug

I need to override username for Proxy provider. Up until 2025.10.* I was using:
```
return {
"preferred_username": request.user.attributes.get("TV_User", request.user.username)
}
```
Now after updating to 2026.2.4 I no longer seem to be able to override username (HTTP_X_AUTHENTIK_USERNAME) in any way.
I also tried:
```
username_override = request.user.attributes.get("TV_User", request.user.username)
return {
"ak_proxy": {
"user_attributes": {
"additionalHeaders": {
"X-authentik-username": username_override
}
}
}
}
```
Also without luck. It also seams that the scope is never actually called. If I added log `ak_logger.info("proxy_override_mapping", user=request.user.username, tv_user=request.user.attributes.get("TV_User", "N/A"))` to mapping the log did not appear while accessing proxy.

### How to reproduce

Set scope mappings. Apply them to Proxy Provider.

### Expected behavior

HTTP_X_AUTHENTIK_USERNAME should be overridable by scope mappings.

### Screenshots

_No response_

### Additional context

_No response_

### Deployment Method

Docker

### Version

2026.2.4

### Relevant log output

```shell

```