[App Service] az webapp ssh silently exits on macOS with Python 3.14 and Invoke 2.2.0

### Describe the bug

On macOS with Azure CLI 2.89.1 installed by Homebrew, `az webapp ssh` establishes the App Service tunnel and opens a Paramiko SSH channel, but immediately closes without displaying an error. The command exits with status 0.

With `--debug`, the interactive session reports `buffer overflow`. Azure CLI catches the exception in `_start_ssh_session`, logs it only at INFO/debug level, closes the connection, and returns success.

The bundled Invoke 2.2.0 uses a 4-byte `"HH"` buffer for `TIOCGWINSZ`. Python 3.14 detects the 8-byte write on macOS and raises `SystemError: buffer overflow`. Updating Invoke to 2.2.1 fixes the command.

Upstream: https://github.com/pyinvoke/invoke/issues/1038
Python 3.14 tracking: https://github.com/Azure/azure-cli/issues/32869

### Related command

az webapp ssh --resource-group --name

### Errors

No error appears normally; the command silently returns with exit status 0.

With `--debug`:

```
paramiko.transport: Secsh channel 0 opened.
paramiko.transport: [chan 0] EOF sent (0)
azure.cli.command_modules.appservice.custom: buffer overflow
Client disconnected
websocket close: Connection failure.
az_command_data_logger: exit code: 0
```

### Issue script & Debug output

``shell
az webapp ssh \
--resource-group \
--name \
--debug
```

The tunnel and SSH channel are established, followed by:

```
paramiko.transport: Secsh channel 0 opened.
paramiko.transport: [chan 0] EOF sent (0)
azure.cli.command_modules.appservice.custom: buffer overflow
Client disconnected
websocket close: Connection failure.
az_command_data_logger: exit code: 0
```

Invoke 2.2.0's `invoke/terminals.py` contains:

```python
fmt = "HH"
buf = struct.pack(fmt, 0, 0)
result = fcntl.ioctl(sys.stdout, termios.TIOCGWINSZ, buf)
```

Invoke 2.2.1 uses the correct 8-byte `"HHHH"` structure. Installing 2.2.1 into Azure CLI's environment makes the same command work.

### Expected behavior

The command should maintain an interactive SSH session. If terminal creation fails, Azure CLI should display the exception and return a nonzero status instead of silently closing with status 0.

### Environment Summary

azure-cli 2.89.1
core 2.89.1
telemetry 1.1.0

Installation: Homebrew
Python location: /opt/homebrew/Cellar/azure-cli/2.89.1/libexec/bin/python
Python: 3.14.7
Bundled Invoke: 2.2.0
Fabric: 3.2.2
OS: macOS 26.6.2
Architecture: arm64

### Additional context

Homebrew's current Azure CLI 2.89.1 formula depends on Python 3.14 and packages Invoke 2.2.0.

Working dependency update:

```shell
"$(brew --prefix azure-cli)/libexec/bin/python" \
-m pip install --ignore-installed "invoke==2.2.1"
```

Another workaround is `az webapp create-remote-connection` plus the system SSH client.

Suggested fixes:
1. Package a Python 3.14-compatible Invoke version.
2. Do not suppress the session exception and report success when SSH failed.