Synapse errors establishing IPv6 appservice connections

### Description

I'm running heisenbridge, and wanted to have synapse connect to it from an IPv6 address (I use IPv6 addressing internally, and only use IPv4 on the edge of my network), but this ended up breaking the bridge (which previously was listening on 127.0.0.1). Turns out that synapse refuses to connect to appservices which contain colons in their url:

> 2025-07-30 16:12:26,509 - synapse.appservice.api - 412 - WARNING - as-recoverer-6 - push_bulk to http://[fdXX:XXXX:XXXX:XXXX::XXXX]:9898 threw exception(InvalidCodepoint) Codepoint U+003A at position 5 of 'fdXX:XXXX:XXXX:XXXX::XXXX' not allowed args=("Codepoint U+003A at position 5 of 'fdXX:XXXX:XXXX:XXXX::XXXX' not allowed",)

### Steps to reproduce

- run any appservice with an IPv6 listen address

### Homeserver

zettaport.com

### Synapse Version

{"server":{"name":"Synapse","version":"1.134.0"}}

### Installation Method

pip (from PyPI)

### Database

PostgreSQL

### Workers

Single process

### Platform

```
# uname -sr
OpenBSD 7.7
```

### Configuration

Nothing unusual besides the IPv6 appservice url.

### Relevant log output

```shell
See above
```

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

_No response_