Terminal reporting not working: 'not a tty'

### Description

`tty` and `ps` commands fail to detect terminal when run under gVisor.
This is easiest to show with Docker, but if `runsc` is run directly with the `"terminal": true` in the config, it has the same problem.

```
$ docker run --runtime=runsc -it --rm ubuntu tty
not a tty
```
```
$ docker run --runtime=runsc -it --rm ubuntu ps t
PID TTY STAT TIME COMMAND
1 ? Rs 0:00 ps t
```

Compare to the output with the default Docker runtime:

```
$ docker run -it --rm ubuntu tty
/dev/pts/0
```
```
$ docker run -it --rm ubuntu ps t
PID TTY STAT TIME COMMAND
1 pts/0 Rs+ 0:00 ps t
```

### Steps to reproduce

_No response_

### runsc version

```shell
runsc version release-20260608.0
spec: 1.2.1
```

### docker version (if using docker)

```shell

```

### uname

_No response_

### kubectl (if using Kubernetes)

```shell

```

### repo state (if built from source)

_No response_

### runsc debug logs (if available)

```shell

```