MAS doesn't support homeserver names with a colon

**Describe the bug**

MAS v0.16.0 doesn't support homeserver names that includes a colon. Additionally, the config check passes with this config, doctor errors.

```
$ podman exec -ti synapse-podman-mas mas-cli config dump | grep -A5 matrix:
matrix:
kind: synapse
homeserver: localhost:8448
secret: secret
endpoint: http://synapse:8448/

$ podman exec -ti synapse-podman-mas mas-cli config check
2025-05-07T14:12:35.232244Z INFO mas_cli::commands::config:92 Configuration file looks good

$ podman exec -ti synapse-podman-mas mas-cli doctor
2025-05-07T14:12:38.575103Z INFO mas_cli::commands::doctor:32 💡 Running diagnostics, make sure that both MAS and Synapse are running, and that MAS is using the same configuration files as this tool.
Error: The homeserver host in the config (`matrix.homeserver`) is not a valid domain.
See {DOCS_BASE}/setup/homeserver.html

Caused by:
invalid international domain name
```

Same error with `matrix.homeserver` set to `example.com:8448`, `localhost:8448`, `1.2.3.4:8448`, or `[1234:5678::abcd]:8448`

**To Reproduce**

Configure a homeserver name including a colon

**Expected behaviour**

Server names including a port number and IPv6 are supported and valid according to the Matrix specification