`register_new_matrix_user` script fails if homeserver uses UNIX socket listener

### Description

If homeserver uses UNIX socket listener, then `register_new_matrix_user` script fails

### Steps to reproduce

- set up homeserver with UNIX socket listener instead of HTTP listener
- try to use `register_new_matrix_user` script
- script fails because of error below

### Homeserver

donotclickthis.link

### Synapse Version

1.145.0rc3 (b=develop,3f2887cf8)

### Installation Method

Other (please mention below)

### Database

PostgreSQL, single PostgreSQL server, I haven't previously used `portdb` script nor previously restored database from backup

### Workers

Single process

### Platform

OrangePi 5 Plus, Armbian 25.5.1 noble, Python 3.14

### Configuration

Presence, MSC2666 and MSC3266 are enabled

### Relevant log output

```shell
Traceback (most recent call last):
File "/home/synapse/synapse/.venv/bin/register_new_matrix_user", line 7, in
sys.exit(main())
~~~~^^
File "/home/synapse/synapse/.venv/lib/python3.14/site-packages/synapse/_scripts/register_new_matrix_user.py", line 300, in main
server_url = _find_client_listener(config)
File "/home/synapse/synapse/.venv/lib/python3.14/site-packages/synapse/_scripts/register_new_matrix_user.py", line 395, in _find_client_listener
return f"http://localhost:{listener['port']}"
~~~~~~~~^^^^^^^^
KeyError: 'port'
```

### Anything else that would be useful to know?

I installed Synapse manually from Git `develop` branch