URL preview not working with raw IPv6 addresses

### Description

Synapse fails to generate URL previews for valid IPv6 URLs. When a URL contains a correctly formatted IPv6 literal inside square brackets (for example: `http://[201:fdd3:b236:edda:67f5:ff3c:af8d:4132]/`), Synapse removes the brackets and attempts to apply IDNA encoding to the raw IPv6 address. Since IPv6 literals contain characters that are not valid in domain names (such as `:`), the idna library raises an `InvalidCodepoint` exception.

According to RFC 3986, IPv6 literals enclosed in `[` and `]` are not subject to IDNA processing and should be treated as numeric IP addresses. Synapse should detect this case and skip IDNA encoding.

### Steps to reproduce

- Try to preview raw IPv6 address:
```
curl 'https://homeserver/_matrix/media/r0/preview_url?url=http://%5B201:fdd3:b236:edda:67f5:ff3c:af8d:4132%5D&ts=1774786740000' -H 'Authorization: Bearer '
```

### Homeserver

Private homeserver

### Synapse Version

1.148.0

### Installation Method

Debian packages from packages.matrix.org

### Database

17.7 (Debian 17.7-0+deb13u1)

### Workers

Multiple workers

### Platform

Debian 13, VPS

### Configuration

```
url_preview_enabled: true
url_preview_ip_range_whitelist:
- '200::/7'
url_preview_ip_range_blacklist:
- "0.0.0.0/0"
```

### Relevant log output

```shell
2026-03-31 16:11:23,750 - synapse.http.client - 402 - DEBUG - Sending request GET http://[201:fdd3:b236:edda:67f5:ff3c:af8d:4132]
2026-03-31 16:11:23,751 - synapse.http.client - 471 - INFO - Error sending request to GET http://[201:fdd3:b236:edda:67f5:ff3c:af8d:4132]: InvalidCodepoint Codepoint U+003A at position 4 of '201:fdd3:b236:edda:67f5:ff3c:af8d:4132' not allowed
2026-03-31 16:11:23,751 - synapse.http.server - 131 - INFO - SynapseError: 500 - Failed to download content: ["idna.core.InvalidCodepoint: Codepoint U+003A at position 4 of '201:fdd3:b236:edda:67f5:ff3c:af8d:4132' not allowed\n"]
```

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

_No response_