No output and hangs up without TTY mode

### Description

gvisor (runsc) does not produce any output and hangs up if TTY mode (ie. `--tty` or `oci.WithTTY`) is not used.

I use containerd directly (either with `ctr` command or with `github.com/containerd/containerd` Go package).

### Steps to reproduce

```sh
ctr run --runtime io.containerd.runsc.v1 docker.io/library/alpine:latest shell-$RANDOM ls -l
```

or

```
package main

import (
"context"
"fmt"
"os"

"github.com/containerd/containerd"
"github.com/containerd/containerd/cio"
"github.com/containerd/containerd/namespaces"
"github.com/containerd/containerd/oci"
specs "github.com/opencontainers/runtime-spec/specs-go"
)

func main() {
if len(os.Args) < 2 {
fmt.Println("Usage: go-ctr-run ")
os.Exit(1)
}

imageName := os.Args[1]
containerName := os.Args[2]
snapshotName := containerName + "-snapshot"
command := os.Args[3:]

ctx := namespaces.WithNamespace(context.Background(), "default") // ---namespace=default

// connect to containerd
client, err := containerd.New("/run/containerd/containerd.sock")
if err != nil {
panic(err)
}
defer client.Close()

// pull image
image, err := client.Pull(ctx, imageName, containerd.WithPullUnpack)
if err != nil {
panic(err)
}

// create a new container
container, err := client.NewContainer(
ctx,
containerName,
containerd.WithImage(image),
containerd.WithNewSnapshot(snapshotName, image),
containerd.WithRuntime("io.containerd.runsc.v1", nil), // --runtime=io.containerd.runsc.v1
containerd.WithNewSpec(
oci.WithProcessArgs(command...),
oci.WithHostNamespace(specs.NetworkNamespace), // --net-host
oci.WithEnv([]string{"TERM=dump", "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"}),
// oci.WithTTY, // --tty
),
)
if err != nil {
panic(err)
}
defer container.Delete(ctx, containerd.WithSnapshotCleanup) // --rm

// create a task
task, err := container.NewTask(ctx, cio.NewCreator(
cio.WithStdio,
// cio.WithTerminal,
))
if err != nil {
panic(err)
}
defer task.Delete(ctx)

// start task
if err := task.Start(ctx); err != nil {
panic(err)
}

// wait for exit
statusC, err := task.Wait(ctx)
if err != nil {
panic(err)
}

// get exit code
status := <-statusC
code, _, err := status.Result()
if err != nil {
panic(err)
}
fmt.Printf("Container exited with code %d\n", code)
}
```

then

```sh
./go-ctr-run docker.io/library/alpine:latest shell-$RANDOM ls -l
```

### runsc version

```shell
runsc version release-20250820.0
spec: 1.2.0
```

### docker version (if using docker)

```shell
containerd github.com/containerd/containerd/v2 2.0.6 991cc3363c290ffd074e069f2b3034c7286ecbe0

or

containerd github.com/containerd/containerd 1.7.27
```

### uname

6.12.40-64.114.amzn2023.x86_64 or 6.8.0-83-generic

### kubectl (if using Kubernetes)

```shell

```

### repo state (if built from source)

_No response_

### runsc debug logs (if available)

I'm not sure how to enable debug for shim (I'm not using CRI!)

containerd with debug mode:

```
time="2025-10-06T11:21:16.742479944Z" level=debug msg="Ignoring events in namespace - \"default\""
time="2025-10-06T11:21:16.749041208Z" level=debug msg="prepare snapshot" key=bash-26343-snapshot parent="sha256:418dccb7d85a63a6aa574439840f7a6fa6fd2321b3e2394568a317735e867d35" snapshotter=overlayfs
time="2025-10-06T11:21:16.756494829Z" level=debug msg="get snapshot mounts" key=bash-26343-snapshot snapshotter=overlayfs
time="2025-10-06T11:21:16.850869950Z" level=info msg="connecting to shim bash-26343" address="unix:///run/containerd/s/894e1ca2baeed4e5ba38bea27f2860a20948a993d49db52c4d7662973e98e99c" namespace=default protocol=ttrpc version=2
time="2025-10-06T11:21:16.936539136Z" level=info msg="loading plugin \"io.containerd.event.v1.publisher\"..." runtime=io.containerd.runsc.v1 type=io.containerd.event.v1
time="2025-10-06T11:21:16.936588143Z" level=info msg="loading plugin \"io.containerd.ttrpc.v1.task\"..." runtime=io.containerd.runsc.v1 type=io.containerd.ttrpc.v1
time="2025-10-06T11:21:16.936603028Z" level=debug msg="registering ttrpc service" id=io.containerd.ttrpc.v1.task
time="2025-10-06T11:21:16.936611912Z" level=info msg="loading plugin \"io.containerd.internal.v1.shutdown\"..." runtime=io.containerd.runsc.v1 type=io.containerd.internal.v1
time="2025-10-06T11:21:16.936765347Z" level=debug msg="serving api on socket" socket="[inherited from parent]"
time="2025-10-06T11:21:16.936806074Z" level=info msg="starting signal loop" namespace=default path=/run/containerd/io.containerd.runtime.v2.task/default/bash-26343 pid=1190 runtime=io.containerd.runsc.v1
time="2025-10-06T11:21:16.937075849Z" level=debug msg="Create, id: bash-26343, bundle: \"/run/containerd/io.containerd.runtime.v2.task/default/bash-26343\""
time="2025-10-06T11:21:16.938498574Z" level=debug msg="Executing: [runsc --root=/run/containerd/runsc/default --log=/run/containerd/io.containerd.runtime.v2.task/default/bash-26343/log.json --log-format=json create --bundle /run/containerd/io.containerd.runtime.v2.task/default/bash-26343 --pid-file /run/containerd/io.containerd.runtime.v2.task/default/bash-26343/init.pid bash-26343]"
```